Methods for isolating nucleic acids

ABSTRACT

Compositions, methods and systems are provided for isolating nucleic acids. A polymerase-nucleic acid complex can be formed by mixing a polymerase enzyme comprising strand displacement activity and a mixture of double stranded nucleic acids. Nucleic acid synthesis can then be initiated by the polymerase enzyme to produce a nascent strand complementary to the first strand, thereby displacing a portion of the second strand. After halting or reducing the rate of nucleic acid synthesis, a hybridizing a hook oligonucleotide can be used hybridize to the nucleic acid through a capture region on the hook oligonucleotide that is complementary to the displaced portion of the second strand. The nucleic acid can then be isolated from the mixture of nucleic acids using the hook oligonucleotide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/007,743, filed Jun. 13, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/270,727, filed Sep. 20, 2016, now U.S. Pat. No.10,000,805, which is a continuation of U.S. patent application Ser. No.14/151,648, filed Jan. 9, 2014, now U.S. Pat. No. 9,475,054, which is acontinuation of U.S. patent application Ser. No. 13/427,725, filed Mar.22, 2012, now U.S. Pat. No. 8,658,364, which claims the benefit ofclaims the benefit of priority to Provisional Application No.61/466,747, filed Mar. 23, 2011, and Provisional Application No.61/531,530, filed Sep. 6, 2011, the full disclosures of which areincorporated herein by reference in their entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED BY U.S.P.T.O. eFS-WEB

The instant application contains a Sequence Listing which is beingsubmitted in computer readable form via the United States Patent andTrademark Office eFS-WEB system, and is hereby incorporated by referencein its entirety for all purposes. The txt file submitted herewithcontains only 3 KB file (Ser. No.01/013,009_2021-05-17_SequenceListing.txt).

BACKGROUND OF THE INVENTION

The ability to read the genetic code has opened countless opportunitiesto benefit humankind. Whether it involves the improvement of food cropsand livestock used for food, the identification of the causes ofdisease, the generation of targeted therapeutic methods andcompositions, or simply the better understanding of what makes us who weare, a fundamental understanding of the blueprints of life is anintegral and necessary component.

A variety of techniques and processes have been developed to obtaingenetic information, including broad genetic profiling or identifyingpatterns of discrete markers in genetic codes and nucleotide levelsequencing of entire genomes. With respect to determination of geneticsequences, while techniques have been developed to read, at thenucleotide level, a genetic sequence, such methods can be time-consumingand extremely costly.

Approaches have been developed to sequence genetic material withimproved speed and reduced costs. Many of these methods rely upon theidentification of nucleotides being incorporated by a polymerizationenzyme during a template sequence-dependent nucleic acid synthesisreaction. In particular, by identifying nucleotides incorporated againsta complementary template nucleic acid strand, one can identify thesequence of nucleotides in the template strand. A variety of suchmethods have been previously described. These methods include iterativeprocesses where individual nucleotides are added one at a time, washedto remove free, unincorporated nucleotides, identified, and washed againto remove any terminator groups and labeling components before anadditional nucleotide is added. Still other methods employ the“real-time” detection of incorporation events, where the act ofincorporation gives rise to a signaling event that can be detected. Inparticularly elegant methods, labeling components are coupled toportions of the nucleotides that are removed during the incorporationevent, eliminating any need to remove such labeling components beforethe next nucleotide is added (See, e.g., Eid, J. et al., Science,323(5910), 133-138 (2009)).

In any of the enzyme mediated template-dependent processes, the overallfidelity, processivity and/or accuracy of the incorporation process canhave direct impacts on the sequence identification process, e.g., loweraccuracy may require multiple fold coverage to identify the sequencewith a high level of confidence.

The present invention provides methods, systems and compositions thatprovide for increased performance of such polymerization basedsequencing methods, among other benefits.

BRIEF SUMMARY OF THE INVENTION

In some aspects, the invention provides a method for isolating apolymerase-nucleic acid complex comprising: forming a polymerase-nucleicacid complex by mixing: (a) a polymerase enzyme comprising stranddisplacement activity, and (b) a nucleic acid comprising a doublestranded portion comprising a first strand and a complementary secondstrand; initiating nucleic acid synthesis by the polymerase enzyme toproduce a nascent strand complementary to the first strand, therebydisplacing a portion of the second strand; halting or reducing the rateof nucleic acid synthesis; hybridizing a hook oligonucleotide to thecomplex through a capture region on the hook oligonucleotide that iscomplementary to at least some of the displaced portion of the secondstrand; and isolating the complex using the hook oligonucleotide.

In some embodiments, the invention further includes loading the isolatedcomplex onto a substrate.

In some embodiments, the invention further includes carrying outsingle-molecule nucleic acid sequencing with the polymerase-nucleic acidcomplex on the substrate.

In some embodiments the nucleic acid is a circular nucleic acid. In someembodiments the nucleic acid comprises a double stranded central regionand single stranded hairpin end regions. In some embodiments the nucleicacid comprises a linear nucleic acid comprising a linear double-strandedadaptor. In some embodiments the hook nucleotide also has a retrievalregion having a sequence that allows the hook nucleotide to be bound andretrieved from a reaction mixture. In some embodiments the retrievalregion is complementary to a sequence attached to beads, and the beadsare used for the isolation of the nucleic acid.

In some embodiments the beads are magnetic beads. In some embodimentsthe retrieval region comprises poly(A), poly(dA), poly(T) or poly(dT).In some embodiments the hook comprises a member of a binding pairproviding for removal of the hook oligonucleotide bound to the nucleicacid in order to isolate the nucleic acid or polymerase-nucleic acidcomplex to which it is bound. In some embodiments the binding paircomprises biotin, digoxigenin, a protein, or an antibody.

In some embodiments halting or rate reduction comprises adding alimiting amount of polymerase synthesis reagents. In some embodimentsthe limiting reagents comprise nucleotides or nucleotide analogs. Insome embodiments halting or rate reduction comprises adding reagents tostop the polymerization reaction after a time.

In some embodiments the capture region comprises a universal captureregion. In some embodiments the capture region comprises a sequence thatis specific for isolation of a desired template molecule. Throughout theapplication, either the tem isolation or the term removal is used toseparating a component from other components in a mixture. For example,in some cases there is removal of the hook oligonucleotide by a bead.The removal of the hook oligonucleotide results in isolation of thecompound to which the hook oligonucleotide is attached. The hookoligonucleotide can be bound to a molecule-of-interest including apolymerase-nucleic acid complex.

In some embodiments a plurality of hook oligonucleotides are added, eachhaving a specific capture sequence that is specific for isolation of apolymerase-nucleic acid complex comprising a desired template molecule.In some embodiments a first hook oligonucleotide having a first capturesequence that is specific for isolation of a polymerase-nucleic acidcomplex comprising desired set of template molecules is added, and thecomplex is isolated, then a second hook oligonucleotide having a secondcapture sequence is added for the isolation of a polymerase-nucleic acidcomplex comprising desired template molecules comprising regionscomplementary to both the first and second sequences. In someembodiments the first strand of the nucleic acid comprises asingle-stranded upstream of the double stranded region, and thepolymerase-nucleic acid complex further comprises a primer complementaryto the single stranded portion of the first strand.

In some aspects, the invention provides a method comprising: fragmentinga double stranded DNA sample into double stranded fragments; ligating toeach end of the double stranded fragments a hairpin to produce apopulation of circular DNA templates having a central double strandedregion and hairpin regions on each end; exposing the population ofcircular DNA templates to a primer complementary to the single strandedportion of a hairpin region of the template and to a DNA polymeraseenzyme having strand displacement activity under conditions in which apopulation of polymerase-template-primer complexes are formed;initiating polymerase mediated DNA synthesis to extend the primer,whereby the primer is extended into the double stranded region,displacing the portion of the double strand; halting or reducing therate of synthesis of DNA; adding to the population of complexes a hookoligonucleotide comprising a capture region complementary to a portionof the double strand under conditions where hybridization occurs; usingthe hook oligonucleotide to isolate the complexes to which ithybridized; thereby isolating complexes having active polymerase enzymefrom complexes that are not active.

In some embodiments the hook nucleotide also has a retrieval regionhaving a sequence that allows the hook oligonucleotide to be bound andretrieved from a reaction mixture. In some embodiments the retrievalregion is complementary to a sequence attached to beads, and the beadsare used for the isolation of the nucleic acid. In some embodiments theretrieval region comprises poly(A), poly(dA), poly(T) or poly(dT). Insome embodiments the hook comprises a member of a binding pair providingfor removal of the hook nucleotide bound to the nucleic acid, resultingin isolation of then polymerase-nucleic acid complex. In someembodiments the binding pair comprises biotin, digoxigenin, a protein,or an antibody.

In some embodiments halting or rate reduction comprises adding alimiting amount of polymerase synthesis reagents. In some embodimentsthe limiting reagents comprise nucleotides. In some embodiments haltingor rate reduction comprises adding reagents to stop the polymerizationreaction after a time. In some embodiments the capture region comprisesa universal capture region. In some embodiments the universal captureregion of the hook oligonucleotide comprises a portion of the singlestranded region of the hairpin and a portion of the double strandedregion of the hairpin that is displaced. In some embodiments the captureregion comprises a sequence that is specific for isolation of complexescomprising a desired template molecule. In some embodiments the captureregion comprises a sequence that is specific for isolation of complexescomprising a desired template molecule.

In some embodiments a plurality of hook oligonucleotides are added, eachhaving a specific capture sequence that is specific for isolation ofcomplexes comprising a desired template molecule. In some embodiments afirst hook oligonucleotide having a first capture sequence that isspecific for removal of a desired template molecule is added, and thecomplex is isolated, then a second hook oligonucleotide having a secondcapture sequence is added for the removal of desired template moleculescomprising regions complementary to both the first and second sequences.

In some embodiments one of the capture sequences is directed to one ofthe template strands, and the other capture sequence is directed to theother template strand. In some embodiments prior to isolating thepolymerase-nucleic acid complexes, a polymerase trap is added to removeuncomplexed polymerase enzyme. In some embodiments the polymerase trapcomprises heparin.

In some aspects, the invention provides a method for DNA sequencingcomprising isolating active complexes, loading the active complexes ontoa substrate such that single complexes can be individually opticallyresolved; exposing the active complexes to a plurality of differentiallylabeled nucleotide analogs whose labels are cleaved upon incorporation,and initiating DNA synthesis and observing each complex to determine thetime sequence of nucleotides that are incorporated.

In some embodiments the substrate comprises an array of zero modewaveguides. In some embodiments the complexes are bound to the substratethrough a biotin-avidin or a biotin-streptavidin linkage.

In some aspects, the invention provides a method for loadingpolymerase-nucleic acid complexes onto a substrate comprising: providinga solution of beads, individual beads having bound thereto a pluralityof polymerase-nucleic acid complexes; exposing the solution to asubstrate comprising coupling groups selective for coupling thepolymerase-nucleic acid complexes to the substrate; and applying a fieldto draw the particles to the substrate and to move the particles acrossthe surface of the substrate, whereby polymerase-nucleic acid complexesbecome bound to the substrate through the coupling groups.

In some cases, rather than having a plurality of polymerase-nucleic acidcomplexes, there is one polymerase-nucleic acid complex for each bead.In this manner, the beads can be used to deposit a singlepolymerase-nucleic acid complex in a given region of the substrate, forexample, one polymerase-nucleic acid complex per zero mode waveguide onthe substrate.

In some embodiments, the invention further includes removing the beadsfrom the substrate, leaving the bound polymerase-nucleic acid complexeson the substrate.

In some embodiments the field is a magnetic, electric, or gravitationalfield. In some embodiments the field to draw the particles to thesubstrate and the field to move the polymerase-nucleic acid complexescomprise different fields. In some embodiments the field comprises amagnetic field.

In some embodiments the magnetic field is applied using one or morepermanent magnets that are moved with respect to the substrate. In someembodiments the magnetic field is applied using one or moreelectromagnets. In some embodiments the substrate comprises an array ofzero mode waveguides. In some embodiments the beads have diameters thatare greater than the diameter of the zero mode waveguide.

In some embodiments, after applying the field, a portion of the zeromode waveguides have a single polymerase-nucleic acid complex attachedthereto.

In some aspects, the invention provides a method for loading activepolymerase-nucleic acid complexes onto a substrate comprising: providinga solution of magnetic beads having polymerase-nucleic acid complexesbound thereto, each polymerase-nucleic acid complex comprising apolymerase enzyme, and a template nucleotide; contacting the solution ofmagnetic beads with the top of a substrate comprising an array ofnanoscale wells having bases, wherein the bases of the wells havecoupling agent bound thereto; and applying a dynamic magnetic field tomove the magnetic beads in solution down to the top of the substrate,whereby the dynamic magnetic field causes the particles to be movedacross the top surface of the substrate, whereby some polymerase-nucleicacid complexes become bound to the coupling groups on the bases of thenanoscale wells.

In some embodiments the polymerase-nucleic acid complexes are bound tothe magnetic bead via hybridization between an oligonucleotide attachedto the magnetic bead and a sequence on the template nucleic acid. Insome embodiments the magnetic bead is attached to a hook oligonucleotidecomprising a retrieval sequence that is complimentary to anoligonucleotide attached to the magnetic bead and a capture sequencethat is complementary to the template nucleic acid. In some embodimentsthe oligonucleotide attached to the magnetic bead comprises a poly(dA),poly(A), poly(dT) or poly(T) sequence.

In some embodiments the relative binding strength of each of i) themagnetic bead to the hook oligo, ii) the hook oligo to the templatenucleic acid, and iii) the polymerase-nucleic acid complex to thesubstrate are controlled such that when the polymerase-nucleic acidcomplex becomes bound to the substrate while applying the dynamicmagnetic field, the attachment between the hook oligo and the templatenucleic acid is broken.

In some embodiments the dynamic magnetic field is produced using one ormore moving permanent magnets. In some embodiments the dynamic field isproduced using one or more electromagnets. In some embodiments thenanoscale wells are cylindrical, and the diameters of the magnetic beadsare greater than the diameter of the nanoscale wells. In someembodiments the coupling agent at the bases of the wells comprisesbiotin. In some embodiments the polymerase enzyme is attached tostreptavidin, neutravidin, or avidin for binding to the coupling agent.

In some aspects, the invention provides an apparatus comprising; afixture for holding a substrate; a substrate held within the fixturecomprising an array of zero mode waveguides on its top surface, andcomprising a reservoir for containing a solution in contact with the topsurface of the substrate; a solution, in contact with the substrate,comprising magnetic beads having polymerase-nucleic acid complexesattached thereto; a device for generating a dynamic magnetic fielddisposed below, adjacent to, or above the substrate capable ofgenerating a magnetic field that (i) pulls the magnetic beads to the topsurface of the substrate and (ii) moves the magnetic beads across thetop surface of the substrate, whereby polymerase-nucleic acid complexesare deposited into the zero mode waveguides on the substrate.

In some embodiments the polymerase-nucleic acid complexes are bound tothe magnetic bead via hybridization between an oligonucleotide attachedto the magnetic bead and a sequence on the template nucleic acid. Insome embodiments the magnetic bead is attached to a hook oligonucleotidecomprising a retrieval sequence that is complimentary to anoligonucleotide attached to the magnetic bead and a capture sequencethat is complementary to the template nucleic acid. In some embodimentsthe oligonucleotide attached to the magnetic bead comprises a poly(dA),poly(A), poly(dT) or poly(T) sequence.

In some embodiments the relative binding strength of each of i) themagnetic bead to the hook oligo, ii) the hook oligo to the templatenucleic acid, and iii) the polymerase-nucleic acid complex to thesubstrate are controlled such that when the polymerase-nucleic acidcomplex becomes bound to the substrate while applying the dynamicmagnetic field, an attachment between the hook oligo and the templatenucleic acid is broken.

In some embodiments the dynamic magnetic field is produced using one ormore moving permanent magnets. In some embodiments the dynamic field isproduced using one or more electromagnets.

In some embodiments the nanoscale wells are cylindrical, and thediameters of the magnetic beads are greater than the diameters of thenanoscale wells. In some embodiments the coupling agent at the bases ofthe wells comprises biotin. In some embodiments the polymerase enzyme isattached to streptavidin, neutravidin, or avidin for binding to thecoupling agent.

In some aspects, the invention provides a method for depositingmolecules-of-interest onto a substrate comprising: providing a solutionof beads wherein each bead comprises a plurality ofmolecules-of-interest linked thereto by a bead to molecule-of-interestlinkage; exposing the solution of beads to a substrate, the surface ofthe substrate comprising binding molecules for binding themolecules-of-interest; using a contacting force to bring the beads intoproximity or into physical contact with the substrate and optionallyusing a distributing force to move the beads across the surface of thesubstrate; and removing the beads from the substrate, thereby producinga substrate having molecules-of-interest bound to its surface throughthe binding molecules.

In some embodiments the bead to molecule-of-interest linkage compriseshybridized oligonucleotides. In some embodiments the hybridizedoligonucleotides have from about 5 to about 40 complementary bases. Insome embodiments the binding molecules comprise biotin, a biotin bindingprotein, an antigen or an antibody. In some embodiments the contactingforce comprises a gravitational, magnetic, electrical, or dielectric, orcentrifugal force. In some embodiments the beads comprise magnetic beadsand contacting and distributing forces comprise magnetic forces.

In some embodiments the substrate comprises an array of nanoscale wellshaving binding molecules on the bases of the wells whereby themolecules-of-interest become attached to the bases of the wells. In someembodiments the molecule-of-interest comprises a protein or a nucleicacid. In some embodiments the molecule-of-interest comprises an enzyme.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the invention for isolating activepolymerase-nucleic acid complexes.

FIG. 2 illustrates the isolation of active complexes using SMRTBell™templates, a hook oligonucleotide, and beads.

FIG. 3 illustrates some potential species in solution after apolymerase-nucleic acid complex is formed, and the polymerase enzyme haswalked-in, and illustrates which of the species can be captured with ahook molecule.

FIG. 4 shows how the invention can be used to isolate active species andto isolate species having selected sequences from a library of nucleicacid fragments.

FIG. 5 illustrates regions of a template nucleic acid that can betargeted for capture by a hook molecule.

FIG. 6 shows a method of the invention for depositingmolecules-of-interest such as polymerase-nucleic acid complexes ontosubstrates such as zero mode waveguide arrays.

FIG. 7 shows some strategies for using reversible stops in a nucleicacid template to control the polymerase walk-in.

FIG. 8 illustrates a method for single molecule sequencing.

FIG. 9 illustrates an apparatus that can be used for single moleculesequencing

FIG. 10(A) shows the structure of a representative hairpin adaptor forproducing SMRTBell™ templates, the structure of a hook oligonucleotidetargeting a portion of the hairpin adaptor. FIG. 10(B) provides a tableshowing some calculated melting temperatures for oligonucleotidehybridization.

FIG. 11 shows a plot of the number of sequencing reads for sampleshaving a mixture of templates illustrating the ability of a specifichook oligonucleotide to capture and enrich a sequence in the presence ofother nucleotides which do not have the sequence.

FIG. 12 is a plot showing how a specific hook molecule can be used toenrich the representation of the template nucleic acid to which it istargeted.

FIG. 13 provides sequencing data showing how magnetic loading ofpolymerase-nucleic acid templates can be more effective than fordiffusion loading.

FIG. 14 shows that bead loading can provide a more even representationof sequence versus the size of the template, where for diffusion,smaller templates tend to be more highly represented.

FIG. 15 shows data indicating that magnetic beads havingpolymerase-nucleic acid complexes attached can be re-used multiple timesto deposit the complexes on substrates.

FIG. 16 shows the results of experiments showing that the reaction ofpolymerase nucleic acids can be halted, the complexes stored, then thereactions re-initiated.

FIG. 17 shows the results of experiments showing how hooks can be usedto enrich complexes having target-specific regions.

FIG. 18 shows experimental results of how magnetic bead loading can beused to load single polymerase-nucleic acid complexes into arrays ofzero mode waveguides.

FIG. 19 are experimental result showing that magnetic bead loadingresults in a larger fraction of longer length templates into zero modewaveguides.

FIG. 20 shows single molecule sequencing results indicating lower levelsof sticking from magnetic bead loaded complexes.

FIG. 21 shows the results of experiments in which the position of themagnet under the chip was varied in order to obtain broad and consistentloading of complexes across a ZMW chip.

FIG. 22 is an outline of a method of using hook oligonucleotides andsingle molecule sequencing in order to measure short tandem repeats.

FIG. 23 outlines a method for using the walk-in of a polymerase enzymeinto a SMRTbell™ template to expose a modified base to a binding proteinor antibody for specific capture.

DETAILED DESCRIPTION OF THE INVENTION

In some aspects, the present invention provides methods and compositionsfor isolating nucleic acids and polymerase-nucleic acid complexes.Isolation generally involves removing desired nucleic acids orpolymerase-nucleic acid complexes from a mixture of other componentsincluding undesired components. In other aspects the invention providesmethods of loading of polymerase-nucleic acid complexes onto substratesfor analysis, for example for nucleic acid sequencing, and in particularfor single-molecule nucleic acid sequencing.

When carrying out single-molecule sequencing, it is often desirable toattach a polymerase-nucleic acid complex to the surface of a substrate,where the attachment is either through the polymerase enzyme or throughthe nucleic acid. When immobilizing these complexes for subsequentsequencing reactions, it is generally desirable that a large fraction ofthe complexes are active, such that when the sequencing reaction iscarried out, the polymerase will carry out nucleic acid synthesis asrequired for sequencing. An approach to ensuring a high active fractionis to prepare polymerase-enzyme complexes in solution, then to perform aprocess to separate the active complexes from those which are inactive.Other approaches toward isolating and purifying the active fraction ofpolymerase-nucleic acid complex is described in copending U.S. PatentApplication entitled Purified Extended Polymerase/Template Complex forSequencing” 61/385,376, filed Sep. 22, 2010, which is incorporatedherein by reference in its entirety for all purposes. We have inventedand describe herein an approach that allows for selection of the activefraction within the sample. In addition to purifying the active fractionof polymerase-nucleic acid, the methods and compositions describedherein allow for the selection and isolation of specific desired nucleicacid molecules within a population of different molecules. The methods,compositions, and apparatuses of the invention also provide improvedmethods of loading template-nucleic acid complexes onto a substrate, forexample for single-molecule nucleic acid sequencing.

The methods and compositions of the invention that are directed toisolating nucleic acids or polymerase-nucleic acid complexes utilize theability of a polymerase having strand displacement activity to open up adouble stranded region to expose a sequence within the region. Thisexposed sequence can then be targeted and captured using a molecule forspecific capture of the sequence. The molecule is referred to herein asa hook molecule. In some cases, capture is performed using anoligonucleotide in the hook molecule that is complementary to thesequence, referred to herein as a hook oligonucleotide. Because anactive enzyme is required to open up the sequence, exposed sequence isonly available for hybridization and capture for those nucleic acidmolecules complexed with an active polymerase. Where apolymerase-nucleic acid complex is inactive, no sequence from thedouble-stranded region is exposed, and therefore no capture takes place.Thus the hook oligonucleotide can be used to capture only thosecomplexes that are active, allowing for the isolation of activecomplexes from those that are inactive.

The methods and compositions of the invention are also directed to theisolation of polymerase enzyme-nucleic acid complexes from othercomponents of a mixture including separating the polymeraseenzyme-nucleic acid complexes from free, uncomplexed enzyme. This can beaccomplished by using a hook molecule designed to capture a targetsequence within the nucleic acid. The hook molecule can be, for example,a hook oligonucleotide that is complementary to a sequence within thenucleic acid in the complex. For this type of capture, the nucleic acidwill generally have both a double stranded and a single strandedportion. A hook molecule can be designed to be complementary to thesingle stranded portion of the nucleic acid, allowing for capturewithout denaturing or opening the double stranded region of the nucleicacid. In some cases, a library of DNA fragments is formed where each ofthe DNA fragments may have a unique sequence, and where each of thefragments has a common, or universal, single stranded region. The use ofa common or universal region allows for capturing many complexes, eachwith regions having different sequences. In other cases, the hook oligocan be targeted to regions within portions of the fragments havingdifferent sequences. This approach can be used for selectively removingnucleic acids from the population which include a desired sequence. Insome cases the single stranded region to which the hook oligonucleotideis complementary comprises a hairpin region of the nucleic acid. Thehairpin regions can be the hairpin regions of SMRT Bell™ templates whichare described in more detail below. The methods that target a common oruniversal sequence in single stranded portion of the population ofnucleic acids will generally not provide a purification of active frominactive complexes the way that the methods described herein includingenzyme walk-in will, but the protocols for this method can be simpler,and still allow for being able to use higher ratios of enzyme to nucleicacid in order to provide higher yields of complex, then providing forremoval of the excess, uncomplexed enzyme. This method also allows forgreater quantitation by measuring the concentration of complex after thepurification from uncomplexed enzyme. The components and conditions,including hook molecules described herein both for methods includingenzyme walk-in and those methods not employing walk-in.

The method can comprise, for example, the following steps. First, apolymerase-nucleic acid complex is formed between a polymerase enzymehaving strand displacement activity, and a nucleic acid comprising adouble stranded portion. The complex usually also includes a primer thatis hybridized to one of the strands of the nucleic acid. Once thecomplex is formed, nucleic acid synthesis is initiated. The synthesis iscarried out under the appropriate conditions generally including thepresence of four nucleotides and the requisite salts, metals, andbuffers. The active polymerase enzymes will produce a nascent strandcomplementary to the first strand, thereby displacing a portion of thesecond strand of the double stranded portion. Before the enzymecompletes the synthesis of the strand, the nucleic acid synthesis ishalted or slowed. This results in the polymerase enzyme being stopped,but remaining part of the sequencing complex.

A hook molecule such as a hook oligonucleotide, having a region that iscomplementary to the displaced and exposed portion of the nucleic acidis then added. The region of the hook oligonucleotide that iscomplementary to the exposed portion of the nucleic acid is referred toas the capture region. In addition to the capture portion, the hookmolecule has a retrieval portion for isolation of the nucleic acid. Theretrieval portion can be an oligonucleotide sequence, a member of abinding pair, or the capture portion can be a solid substrate such as abead or planar surface. The retrieval portion allows for the isolationof the polymerase-nucleic acid complex from other molecules in themixture including inactive complexes. In preferred embodiments, theretrieval portion is a magnetic bead, or is used to attach thepolymerase-nucleic acid complex to a magnetic bead. Thepolymerase-nucleic acid complex can then be separated from the othercomponents of the mixture by well known methods of magnetic beadpurification. The isolated polymerase-nucleic acid complex can then beremoved from the hook molecule for subsequent use, such as for nucleicacid sequencing. For example, where the hook molecule is a hookoligonucleotide that is hybridized to the polymerase-nucleic acidcomplex, the polymerase-nucleic acid complex can be released by raisingthe stringency of the solution, for example by lowering the ionicstrength or raising the temperature.

These methods and compositions can be used to selectively isolate activecomplexes from inactive complexes and other components in the mixtureused to create the complexes. The methods and compositions can also beused to selectively isolate nucleic acids and polymerase-nucleic acidcomplexes having specific sequences from a mixture of nucleic acids. Forexample, for DNA sequencing, genomic DNA can be fragmented into amixture of double-stranded pieces falling within a desired size range.The fragments can be treated, e.g. by ligation of adapters to the endsof the fragments. These adapters can be used as sites for priming andfor formation of polymerase-nucleic acid complexes. With the methodsdescribed herein, hooks to universal sequences can be used to eitherisolate the active fraction from other components in the mixtureregardless of the sequence of the nucleic acid fragment, or specifichooks can be used to selectively remove fragments containing desiredsequences.

The isolated polymerase-nucleic acid complexes can be used for loadingactive polymerase-nucleic acid complex onto substrates for nucleic acid(e.g. DNA) sequencing. In some cases, the polymerase-nucleic acidcomplexes are washed from the hook molecules, e.g. using highstringency, then the solution of polymerase-nucleic acid complexes isadded to a substrate for attachment of the complexes. In some cases, wehave found that beads onto which the polymerase-nucleic acid complexesare attached can be used to load the complexes onto the surface withoutprior removal of the complexes from the beads. For example, magneticbeads having polymerase-nucleic acid complexes bound to them byhybridization can be added to a substrate in the form of a solution, anda magnetic field applied to bring the beads down to the substrate,allowing the complexes to contact the surface of the substrate, andbecome bound to it. We have found that providing a dynamic (as opposedto a static) magnetic field that moves the beads around on the surfaceof the substrate helps in successful binding of the complexes to thesurface. The surface binding can be accomplished by having couplinggroups attached to the surface, and having molecules to which thecoupling groups bind on the polymerase-nucleic acid complexes. In someembodiments, the polymerase will have a biotin binding protein such asstreptavidin, and the surface will have biotin coupling groups attachedto it. The strong chemical interaction between the biotin and thebinding protein will result in the immobilization of the polymerase.Where there is a relatively weak binding between the hook molecule andthe template nucleic acid (e.g. through nucleic acid hybridization) thepolymerase-nucleic acid complex can become bound to the surface and theforce of the magnetic field will displace the hook from the complex,leaving the bound complex on the surface of the substrate. In preferredembodiments, the substrate comprises an array of zero mode waveguides inthe form of nanoscale wells in which the nano scale wells have couplinggroups such as biotin on the bases of the wells, resulting in thedeposition of polymerase-nucleic acid complexes in the zero modewaveguides.

FIG. 1 illustrates an embodiment of the invention for the isolation ofpolymerase-nucleic acid complexes. A nucleic acid having a doublestranded nucleic acid region 100 is provided. The double stranded regionhas a first strand 104, and a second strand 106. In this embodiment, thefirst strand 104 acts as the template strand, and the second strand 106is complementary to this strand. The double stranded nucleic acid 100can be part of a mixture of different sequences, for example from alibrary of nucleic acid fragments. In preferred embodiments, the nucleicacid is DNA, but it can also comprise RNA residues or RNA strands andcan also include non-natural nucleotides. In the embodiment shown inFIG. 1 , the nucleic acid 100 has a double stranded region, and also hasa single stranded region. The single stranded region provides a placefor the hybridization of a primer 112. While a single stranded region isuseful for the invention, it is not absolutely required. In some cases,for example, the nucleic acid in one strand can be nicked to provide astarting point for nucleic acid synthesis, and in some cases terminalprotein can be used to initiate synthesis in the absence of a primer.

In step (I) a primer 112 and a polymerase enzyme 110 are added to formthe polymerase-nucleic acid complex. The appropriate salts, metals,buffers, etc. are added during complex formation. In some cases one ormore natural or unnatural nucleotides is added to stabilize the complexby keeping the polymerase enzyme in a conformation that will remainbound to the nucleotide. As is well known in the art, the polymeraseenzyme is able to identify and bind to the appropriate location at the3′ end of a primer poised for nucleic acid synthesis. In some cases, itis desirable to add an excess of polymerase enzyme in step (I) to ensurea high yield of complex formation. For example, in some cases, molarratios of 10:1 to 50:1 of polymerase enzyme to nucleic acid are used.Being able to use such a large excess of polymerase is an advantage ofthe current method, because the subsequent isolation of the desirednucleic acid complex allows for isolation from the excess addedpolymerase.

In step (II), nucleic acid polymerization is initiated, resulting in theformation of a nascent strand 120 extended from the primer. In order toinitiate polymerization, all of the required components, including allnecessary nucleotides are added to the solution containing the complex.The polymerase enzyme that is used has strand displacement activity.This results in the displacement of the second strand 106 that iscomplementary to the first (template) strand 104, producing a displacedsingle stranded region of the second strand 122.

In step (III) a hook molecule 130 attaches to a sequence that wasexposed by the action of the polymerase enzyme through a capture region134 on the hook molecule. In preferred embodiments, the hook molecule isa hook oligonucleotide having a capture sequence that is complementaryto a sequence on the displaced second strand. The hook molecule also hasa retrieval region 132 that allows the hook attached to thepolymerase-nucleic acid complex to be separated from other components inthe mixture. As shown in FIG. 1 , the retrieval region 132 can be amember of a binding pair that will bind to a surface for retrieval. Inother cases, the retrieval region can be a bead or solid surface.

In step (IV) an affinity substrate 140 having an affinity for theretrieval portion of the hook molecule is added, the substrateselectively binding the retrieval portions of the hook molecules. Theaffinity substrates can comprise beads or planar surfaces that havebound to them agents to bind to the retrieval portion of the hookmolecules. In some cases, the affinity substrates can comprise magneticbeads. In some cases, magnetic beads having poly(T) sequences are usedto hybridize to poly(A) regions on the retrieval portions of the hookmolecules. The use of magnetic beads for separation of biomolecules iswell developed. Once the polymerase-nucleic acids are attached to theaffinity substrates, other components of the solution can be washed awayproviding isolation of the complexes. In addition to washing away ofinactive polymerase-nucleic acid complex, the process can remove othercomponents from the complexation reaction such as removing excessuncomplexed polymerase enzyme. In some cases, a polymerase trap is addedprior to the step of washing away the components of the complex formingreaction. A polymerase trap is used to bind excess free polymerasewithin the reaction in order to more effectively remove it from thedesired polymerase-nucleic acid complex by washing. A useful polymerasetrap is heparin, to which polymerases are known to bind. Nucleic acidssuch as DNA can also be used as polymerase traps to assist in removal ofthe excess polymerase. In some cases, single stranded DNA such ascircular single stranded DNA can be used.

In step (V) the retrieved polymerase-nucleic acid complex is removedfrom the hook molecule and from the affinity substrate. Where the hookmolecule is a hook oligonucleotide having a capture sequence that ishybridized to the polymerase-nucleic acid complex, thepolymerase-nucleic acid complex can be removed from the hookoligonucleotide by treatment with high stringency conditions, e.g. lowsalt concentration and/or higher temperature. The isolation process canprovide for adjusting the concentration of the polymerase-nucleic acidcomplex. For example, where the polymerase-nucleic acid complex isreleased into a volume that is smaller than the volume it was in priorto isolation, the concentration of the polymerase-nucleic acid can beincreased.

The process of FIG. 1 can be used to increase the active fraction ofpolymerase-nucleic acid complex. Since only active polymerases willdisplace the second strand to expose the capture sequence, onlycomplexes having an active polymerase will be isolated using the hookmolecule. Having a higher fraction of active complex can be valuable forsingle-molecule sequencing approaches where maximizing the portion ofactive polymerase-nucleic acid complex can result in a higher yield inthe number of sequencing reactions per substrate. In addition toincreasing the active fraction of complex, the methods allow for theisolation of the complex from other components in the complex-formingreactions such as excess polymerase enzyme.

In preferred aspects of the invention, the template nucleic acid is in acyclic form. Performing single-molecule sequencing on a cyclic nucleicacid template is advantageous in that it allows for redundant sequencingof a given region. The accuracy of a sequence determination can beimproved significantly by sequencing the same region multiple times.Cyclic nucleic acids that are highly useful for the current inventioninclude SMRT Bell™ templates, which are nucleic acids having a centraldouble-stranded region, and having hairpin regions at each end of thedouble-stranded region. The preparation and use of cyclic templates suchas SMRT Bells™, are described for example in U.S. patent applicationSer. No. 12/286,119, filed Sep. 26, 2008, and U.S. patent applicationSer. No. 12/383,855, filed Mar. 27, 2009, the full disclosures of whichis incorporated herein by reference for all purposes. One advantage ofthe SMRT Bell™ template is that it can be made from a library ofdouble-stranded nucleic acid, e.g. DNA, fragments. For example, a sampleof genomic DNA can be fragmented into a library of DNA fragments, byknown methods such as by shearing or by use of restriction enzymes. Thelibrary of DNA fragments can be ligated to hairpin adaptors at each endof the fragment to produce a library of SMRT Bell™ templates. Thehairpin adaptors provide single stranded regions within the hairpins. Byusing the same hairpin adaptor for all of the fragments, the hairpinadaptors provide a position for universal priming of all of thesequences.

FIG. 2 shows an embodiment of the invention using circular, SMRT Bell™templates. The SMRT Bell™ templates 200 are provided having a centraldouble stranded region 204 and having hairpin regions 202 on the ends ofthe double stranded region. In some cases, the SMRT Bell™ can comprise alibrary of different sequences in the double-stranded region 204. Instep (I), a primer 212 is added that is designed to hybridize to thesingle stranded region of the hairpin region of the template nucleicacid. A polymerase enzyme 210 having strand-displacement activity isalso added under conditions in which the polymerase enzyme will form acomplex with the template and primer.

In step (II) template directed nucleic acid synthesis is initiated byadding the required reagents under the appropriate conditions. Thereagents for carrying out nucleic acid synthesis are well known andinclude nucleotides, salts, and metals in solution at the appropriateconcentrations and pH. As the polymerase enzyme extends the primer, itmoves into the double stranded region, displacing the nucleotide strandthat is complementary to the strand it is using as a template forsynthesis. The polymerase enzyme is then halted or the rate of synthesisis reduced. The rate is reduced such that few, if any, nucleotide unitswill be added during the subsequent steps of the method. For example,the rate of enzyme synthesis is reduced by a factor of 10, 100, or 1000.In some cases, the enzyme can be halted by adding a limiting amount ofreagents. The enzyme can also be halted by allowing the synthesis toproceed for a given amount of time, and then stopping the synthesis byadding an agent that halts synthesis, such as Sr++. We have found thatin some cases, it is useful to run the nucleic acid synthesis in thepresence of Ca++. Having Ca++ as the divalent metal in the reaction, andvery little to no Mg++ or Mn++, the polymerization proceeds, but quiteslowly, and in a controlled fashion, allowing for controlling the amountof walk-in before halting synthesis. In some cases, the halting orslowing of rate is accomplished by buffer exchange. The walk-in can alsobe controlled by including modified nucleotide residues within thetemplate nucleic acid that act to halt or significantly slow nucleicacid synthesis. The modifications which lead to the halting of thesynthesis can be made in a reversible fashion, allowing them to beremoved for subsequent sequencing reactions. The reversiblemodifications can be removed by light or by the addition of reagents fortheir specific removal.

After the polymerase reaction is halted, in step (III) a hook moleculeis added. The hook molecule 230 has a capture region that is specificfor binding to the portion of the nucleic acid that was exposed uponnucleic acid synthesis and opening up of the double stranded region. Inthe embodiment of FIG. 2 , the hook molecule is a hook oligonucleotidehaving a capture region that is complementary to and will hybridizespecifically with the exposed portion of the nucleotide. In theembodiment of FIG. 2 , the hook oligonucleotide capture region has aportion that hybridizes to the single stranded region of the hairpin,and a portion that hybridizes to the previously double-stranded portionthat was opened up by the action of the polymerase enzyme. In somecases, the capture region of the hook oligonucleotide will hybridizeonly to the exposed former double stranded region. The hookoligonucleotide also has a retrieval region, which in the embodiment ofFIG. 2 comprises a poly(A) (or poly(dA)) sequence.

In step (IV) beads 250, such as magnetic beads having retrieval moietiessuch as a plurality of poly(T) oligonucleotides 240 bound to theirsurfaces are added to the mixture. The poly(T) regions on the beadshybridize with the poly(A) regions on the hook oligonucleotide,specifically trapping onto the beads only the polymerase-nucleic acidcomplexes bound to the hook oligonucleotide. In the embodiment describedhere, beads are used. Other solid substrates can also be used, forexample the retrieval moities can be part of a column, a filter medium,or can be attached to a flat solid substrate such as a hybridizationmicroarray. During the trapping the stringency of the solution iscontrolled in order to favor capture, for example by controlling theionic strength and the temperature of the solution. Once the capturedcomplexes are attached to the beads, the beads can be washed to removeother components within the reaction mixture. For example excesspolymerase and un-complexed nucleic acid and primer can be removed. Inaddition, complex that was formed, but was not active, will not haveexposed regions to be captured by the hook molecule, so these inactivecomplexes can be removed. In some cases, a hook having a specificcapture region designed to hybridize with nucleic acids having a desiredsequence is used. In these cases, the washing will also remove complexesthat do not have the desired sequence even where the complex is active.

In step (V), the desired active polymerase-nucleic acid complex isremoved from the hook molecule and the bead. Where a hookoligonucleotide is used, removal can be accomplished by raising thestringency of the solution such that the capture region of the hookoligonucleotide no longer hybridizes to the complex. The concentrationof the complex that is isolated can be controlled by controlling thevolume into which the oligonucleotide is eluted. By keeping this volumelow, the concentration of the complex can be kept relatively high. Insome cases, the process can be used to concentrate the isolated complexby using volumes smaller than the original volume into which the complexwas dissolved. The complex can also be removed from the beads byphysical means such as contacting the beads with a substrate havingbinding groups that will bind the complex strongly and sever the bond tothe bead. In some cases, the beads physically contact the surface of thesubstrate, in other cases, the beads are brought into proximity with thesubstrate.

FIG. 3 illustrates some of the species that can be formed in preparingthe polymerase-nucleic acid complex and that can be removed using thehook molecule. A polymerase-nucleic acid complex is formed between aSMRTBell™ nucleic acid 306, a polymerase enzyme 304, and a primer 302.As described above, a nucleic acid synthesis reaction is carried out instep (I) to allow the polymerase to walk in, extending the primer toform a nascent strand, and displacing the non-template portion of thedouble stranded region. The nucleic acid synthesis is halted after theenzyme has replicated a portion of the nucleic acid. At this point, thereaction mixture may have a number of different species. Some of thespecies are: free, uncomplexed enzyme 310, active polymerase-nucleicacid complex 320, and inactive polymerase-nucleic acid complex 330. Insome cases the hairpin regions on each end of the SMRT Bell™ templatehave the same sequence. Where this is the case, some complex having twopolymerase enzymes can be formed. In this case the reaction mixture mayalso have complex with one active enzyme 340, and complex with twoactive enzymes 350. In some cases, a different hairpin region is at oneend of the double stranded region than on the other end. Where thehairpin regions are different, the formation of species 340, and 350 canbe avoided. In addition, in some cases the reaction mixture can havecomplex in which two active enzymes have completely replicated each ofthe strands in the central double stranded region 360. This can occurfor example, where the double stranded region is relatively short. Forthe species 360, the region of the nucleic acid that was originallyexposed by the enzyme becomes double stranded again as the enzymecompletes replication of the strand. These species, while being active,will not be retrieved in the later part of the process.

This illustrates an aspect of the invention which accomplishes sizeselection. By controlling the amount of walk-in with a circulartemplate, one can exclude the capture of species that are shorter than acertain length by walking in a distance such that the template is fullyreplicated, because when a circular template is fully replicated, thecapture portion of the sequence is double stranded and thereforeinaccessible to the hook. This replication of the complete template canbe done with two enzymes per template as shown in FIG. 3 , or can bedone with a single enzyme per template. This can be useful, for example,in cases where short nucleic acids are not desired, which can be thecase in sequencing, where longer readlengths are often desired, andwhere very short fragments are relatively non-productive. It can also beuseful for the removal of hairpin adaptor dimers, which will have a veryshort double stranded region.

In step (II) the hook molecule 370 is added to the mixture. As shown,the hook molecule 370 will bind to the species in which the activity ofthe polymerase has opened up the double stranded region to expose thesequence to which the hook molecule, e.g. hook oligonucleotide isdesigned to bind. The hook molecule binds species 320, 340, and 350,which have the requisite exposed regions, but does not bind freepolymerase 310, inactive complex 330, and small nucleic acids in whichboth of the strands in the double stranded region are fully replicated360. The hook molecule can thus be used, for example by binding to asolid surface and washing, to isolate the bound species from those whichare unbound and to isolate bound species from other components of themixture.

FIG. 4 illustrates a process for isolating active complexes from adouble-stranded nucleic acid sample. A double stranded nucleic acidsample, such as a genomic DNA sample, is fragmented in step (I) into alibrary of fragments. Fragmentation can be carried out in any suitablemanner, including shearing and restriction fragmentation. The library offragments can then be treated to produce ends that are amenable tofurther processing. In some cases, enzymes can be added that produceblunt ends. In some cases, the ends will have overhang regions.

In step (II), hairpin adaptors 410 are added to the fragments andligated onto the ends to produce a SMRTBell™ construct having a centraldouble stranded region and hairpin regions at each end. The hairpinadaptors 410 are oligonucleotides which generally have both a singlestranded hairpin region and a double stranded region. As shown here, thehairpin adaptor can function as a universal sequence allowing a singleprimer to initiate synthesis on all of the fragments even though thefragments can have different sequences. In addition, for the purposes ofthe present invention, the double stranded region can act as a universalregion, providing a capture sequence for attachment to the captureportion of a hook oligonucleotide, allowing all nucleic acids that havethis portion of the hairpin adaptor sequence exposed to be captured,regardless of the sequence that derives from the fragment.

In some cases, the hairpin adaptors at each end of the double-strandedcentral region will be the same. In other cases, a hairpin adaptorcomprising one sequence is provided at one end of the double strandedsegment, and a hairpin adaptor with a different sequence is provided atthe other end. Having different hairpin adaptors at each end generallyentails using a more involved protocol, but in some cases, havingseparate adaptors at each end can be advantageous.

In step (III) nucleic acid synthesis is initiated, such that thepolymerase enzyme extends the synthesis of the nascent strand into thedouble stranded region, exposing a portion of the strand that is notacting as the template. The polymerase reaction is then halted. Theresulting polymerase-nucleic acid complexes can then be isolated fromthe reaction mixture using hook molecules. In some cases it is desiredto isolate all active complexes regardless of sequence, in which casethe sequence on the hairpin adaptor which is universal for all fragmentscan be targeted by the capture region of the hook molecule. In somecases, it is desired to select only active complexes having a specificsequence, in which case, a hook molecule having a capture regiontargeted only to that sequence can be used. This allows for theisolation of only those nucleic acids within the mixture of fragmentswhich have the sequence of interest. Multiple specific hook moleculescan be added where it is desired to isolate nucleic acids having any oneof a set of sequences targeted by the multiple specific hook molecules.

In some cases hook sequences can be made to target sequences that arenot desired, e.g. for background knockdown. There are situations, forexample in DNA sequencing in which there are contaminating sequencesthat it is known are not desired and will use up useful sequencingresources. For example, in some cases, hook molecules can be used totarget sequences representing housekeeping genes in order to removethese from the mixture. Thus, in some embodiments, hook oligonucleotidesfor capturing both desired and undesired sequences will be deployed,with the undesired sequences separated from those desired. This can bedone sequentially, e.g. by first exposing the sample to hookoligonucleotides having the undesirable sequences, separating thosebeads from the sample, then in a second step exposing the sample to hookoligonucleotides targeted to the desired sequences. In some cases, hookshaving desired and undesired sequences can be added at the same time.For example the hook molecules having undesired sequences can beattached to non-magnetic beads, and the hook molecules having desiredsequences attached to magnetic beads, allowing for selective removal orisolation of only the desired sequences by magnetic isolation.

FIG. 5 illustrates how the hook capture region can be used to targetdifferent portions of the nucleic acid comprising the polymerase-nucleicacid complex. Each of FIGS. 5A-5D shows a polymerase-nucleic acidcomplex comprising a SMRTBell™ type template, in which the polymerasehas walked in, producing a nascent strand extending into the formerlydouble stranded region of the nucleic acid. In FIG. 5A, a capture regionis directed to a sequence that spans both the hairpin region and theformerly double-stranded region. This type of capture region is usefulfor universal capture of all of the active complexes in a mixture ofnucleic acids. While a portion of the sequence to which the captureregion is directed is in the single stranded region, the length of thecapture region, e.g. capture oligonucleotide region, is designed suchthat hybridization to only the single stranded portion will not providea tight enough interaction in order to capture the nucleic acid,requiring capture to an exposed double stranded region for removal orisolation.

In FIG. 5B, the capture portion of the hook molecule is directed only atthe portion of the hairpin adaptor that was formerly double stranded.This approach can be taken where the double stranded region of thehairpin adaptor is long enough to provide a sequence having adequatebinding for capture. In FIG. 5C, the capture portion of the hookmolecule is directed to the portion of the nucleic acid from the doublestranded fragment, and is not directed to the hairpin adaptor portion ofthe nucleic acid. This approach allows for the capture of only thenucleic acids within the mixture that contain the specific sequence tobe captured. FIG. 5D illustrates that in some cases, multiple sequenceswithin the double stranded region can be targeted. In FIG. 5D, hookoligonucleotide 510 is directed to a sequence on one strand of thefragment, and hook oligonucleotide 520 is directed to a sequence on theother strand of the fragment. In some cases, multiple hookoligonucleotides can be directed to different sequences on the samestrand of the fragment. Multiple hooks can be used, for example, in asequential manner, whereby one hook molecule is used to isolate nucleicacids having a given sequence, then a second hook molecule is used toisolate from that set of nucleic acids the nucleic acids having a secondsequence. The polymerase-nucleic acid complexes isolated after thisprocess will be those nucleic acids having both sequences of interest.This process can be extended beyond two sequences for the isolation ofsequence having 3, 4, 5, or more sequences.

Once the active polymerase-nucleic acid complexes are attached to beads,we have found that these complexes can be bound onto a substratedirectly by contacting the beads with the substrate or by bringing thebeads into close proximity with the substrate. This approach allows forloading the complexes onto the substrate without going through aseparate step of releasing the complexes from the beads into solution.

FIG. 6 shows an embodiment of the invention for loadingpolymerase-nucleic acid complexes onto a substrate directly from beads.A substrate 610 is provided. The substrate will generally have couplinggroups that will react with moieties on the polymerase-nucleic acidcomplexes to bind the complexes to the surface. In the embodiment ofFIG. 6 , the substrate comprises an array of nanoscale wells or zeromode waveguides 616. The zero mode waveguides 616 on substrate 610 arenanoscale apertures through a cladding layer 614 that has been depositedonto a transparent substrate 612. The thickness of the cladding layer isgenerally from about 10 nm to about 300 nm. The zero mode waveguides canbe, for example, cylindrical holes having diameters from about 10 nm toabout 300 nm. Such zero mode waveguide arrays can be used for singlemolecule analysis such as single molecule sequencing as describedherein. The zero mode waveguide can be in any suitable shape includingcylinders or cones. The shape can be a channel. The cross sectionalshape of the zero mode waveguide can be a circle, a triangle, a square,a rectangle, or an ellipse, or the cross sectional shape can be anarbitrary shape. For performing analysis within zero mode waveguides itis often desirable to have immobilized molecules-of-interest bound tothe base of the zero mode waveguide, but to have little to substantiallyno molecules-of-interest on other parts of the substrate. Methods fortreating the surfaces of zero mode waveguides including methods forobtaining selective coupling to the base of the zero mode waveguides aredescribed, for example, in U.S. Pat. Nos. 7,833,398, 7,292,742 and inU.S. patent application Ser. No. 11/731,748, filed Mar. 29, 2007, Ser.No. 12/079,922, filed Mar. 27, 2008, and Ser. No. 12/074,716, filed Mar.5, 2008, the full disclosures of which are incorporated by referenceherein for all purposes. In some cases, for example, biotin isselectively coupled to the base of the zero mode waveguide.

Onto the substrate is dispensed a solution of beads 602 havingmolecules-of-interest, e.g. polymerase-nucleic acid complexes 604 boundto them. The complexes will generally have a binding moiety that willattach to the coupling group deposited onto the substrate surface. Forexample, where the substrate comprises biotin coupling groups, a biotinbinding protein can be bound to the polymerase-nucleic acid complex. Thebiotin binding protein can be, for example, streptavidin that is boundto the polymerase enzyme. These polymerase-nucleic acid complex coatedbeads can be made in any suitable manner. In some cases they are madeusing the hook molecule method described herein. The solution comprisingthe beads 602 is generally an aqueous solution having the componentsrequired for keeping the polymerase-nucleic acid complex together. Thebeads 602 can be magnetic beads. The size of the beads will depend onthe application. In some cases, it is desirable that the beads have adiameter that is larger than the diameter of the zero mode waveguide.

In step (I), the beads are brought into contact with the substrate. Thiscan be accomplished, for example, by applying a field that causes thebeads to move down onto the top of the substrate. Where the beads aremagnetic, a magnetic field can be used to draw the beads down. Inaddition to drawing the beads down, we have found that it can bedesirable to provide a dynamic field that causes the beads to moveacross the top of the substrate. This can be accomplished, for example,by moving a permanent magnet under the substrate in a manner that causesthe beads to move. One or more permanent magnets can be moved in arotary fashion such that the beads are swept across the surface. Inother cases, one or more fixed electromagnets provided with varyingcurrents can be used to produce the dynamic field. In general, beads arereferred to as magnetic beads where a magnetic field can be used to movethe beads.

In step (II) the beads are removed from the substrate surface. Wheremagnetic beads are used, this removal or isolation can be performed byusing magnets to the side and from above the sample.

We have found that this process can result in the attachment within thezero mode waveguides of polymerase-nucleic acid complexes. By using thisprocess, we have found that we can achieve the same levels of loading aswith diffusion loading of polymerase-nucleic acid complexes in solutionwith a much smaller amount of complex. We have seen equivalent loadinglevels for bead assisted loading using complex amounts in solution thatare more than an order of magnitude less than for diffusion loading. Themethods of the invention can similarly be used to attach othermolecules-of-interest, for example biomolecules.

We have determined from these experiments that the attachment betweenthe polymerase-nucleic acid complex and the bead is broken during theprocess, leaving the complex bound to the surface while the beads areremoved. There can be several places where the break in the attachmentof the complex to the bead can occur. One aspect of the invention iscontrolling the place at which the break occurs by designing into theconstruct linkages having appropriate levels of binding. Various typesof linkages are possible, and some types have stronger binding thanothers. In some embodiments of the invention, a nucleic acidhybridization is used as the weakest link in the chain of binding. Insome cases two or more hybridization linkages can occur in the chain ofbinding, and one can be made to be stronger than another, for example byhaving a longer region of sequence homology. The strength of the linkagecan also be controlled by including modified or non-natural bases, e.g.peptide nucleic acids (PNAs), adding mismatched bases, and by changingthe conditions in the solution including ionic strength and/or thetemperature.

One example of controlling the position of the break in the linkagebetween the bead and the complex is provided where the polymerase enzymeis bound to the surface via a biotin-streptavidin linkage, thepolymerase enzyme is bound to the nucleic acid by an enzyme-substrateinteraction at the active site, the nucleic acid is bound to a hookoligonucleotide by hybridization with a capture sequence on the hookoligonucleotide to a sequence on a hairpin adaptor portion of thenucleotide of about 10 to about 15 base pairs, and the hookoligonucleotide is attached by hybridization from a retrieval region onthe hook oligonucleotide of about 18 to about 30 nucleotides to anoligonucleotide attached to a magnetic bead, e.g. with a poly(dA) regionon the hook oligonucleotide and a poly(dT) region on the magnetic bead.For this type of construct, we have determined that the hybridizationlinkage between the capture region of the hook and the nucleic acid isthe weakest link that is most susceptible to breaking during themagnetic bead loading. Having breakage at this locus is advantageous, asit leaves the polymerase-nucleic acid complex on the surface without thehook or any portion of the bead attached to it. It is interesting tonote that the binding between the polymerase and the nucleic acid isstronger than the oligonucleotide hybridization linkage under theappropriate conditions. We have found by labeling experiments that boththe polymerase and the enzyme remain on the substrate, showing that thisbond remains as the hook-oligonucleotide bond is broken.

The bead loading described herein can be used for loading of singlecopies of desired molecules into zero mode waveguides. In some cases,obtaining a substrate with a relatively high level of single moleculesfor observations can be accomplished using an appropriate level ofdilution, and relying on a statistical distribution. For example, asample can be diluted and loaded such that on average, some regions haveno polymerase, some have a single polymerase, and some have multiplepolymerases. This type of process results in loading levels that can bemodeled as a Poisson distribution. This type of statistical loading canalso be accomplished with the bead loading of the invention. Therelative amount of active polymerase-nucleic acid complex bound to thebeads can be varied by varying the concentration at which the beads areloaded. We have found that by loading beads at various concentrations,we can obtain a bead loading that provides numbers of single moleculeson the substrate that are consistent with a Poisson distribution. Wehave found that the amount of material required to loadpolymerase-nucleic acid complexes onto a zero mode waveguide substratefor bead loading can be more than an order of magnitude less thanrequired for solution loading.

Methods for Isolating Active Polymerase-Nucleic Acid Complexes

In some aspects the invention provides for methods and compositions forisolating active polymerase-nucleic acid complexes. Hook molecules suchas hook oligonucleotides are used to capture and retrieve activepolymerase-nucleic acid complexes by capturing one or more sequencesthat are exposed only after the action of the polymerase in the complexof opening up a double-stranded region to make the sequence available tothe hook molecule. Isolating active complex from other components in themixture can be useful for nucleic acid sequencing and particularly forsingle molecule sequencing. The complexes can be loaded onto a substratesuch as a zero mode waveguide array for single molecule sequencing, andthe methods of the invention allow for obtaining higher qualitysequencing data by ensuring that a purer sample with a higher level ofactive complex is loaded onto the surface. In addition to isolating theactive complexes, the methods can also be used to selectively isolatespecific nucleic acids of interest.

The methods of the invention include a process comprising: first forminga polymerase-nucleic acid complex by mixing: (a) a polymerase enzymecomprising strand displacement activity, and (b) a nucleic acidcomprising a double stranded portion comprising a first strand and acomplementary second strand. The complex will generally have a primingsite, to which the polymerase will tend to migrate. One or more primerscan be added to form the priming site. Once the complex is formed,nucleic acid synthesis by the polymerase enzyme in the complex isinitiated to produce a nascent strand complementary to the first strand.The synthesis of the nascent strand complementary to the first strand bythe strand displacing enzyme results in the displacement a portion ofthe second strand. The amount of synthesis that occurs, that is, thenumber of bases that are added by the polymerase, is controlled byhalting the nucleic acid synthesis in a controlled manner. This can bedone, for example, by carrying out the nucleic acid synthesis underrelatively slow, controlled conditions and stopping the synthesis at aspecific time, by limiting the reagents such as the nucleotides, or byengineering stopping points into the nucleic acid. The conditions areselected such that the polymerase-nucleic acid complex is stable. A hookmolecule such as a hook oligonucleotide is then added. The hook moleculehas a capture region that is designed to specifically bind to adisplaced portion of the second strand. In preferred embodiments thecapture region comprises an oligonucleotide having a sequence that iscomplementary to a sequence in the displaced portion of the secondstrand. This hook molecule can be used to isolate the complex to whichit is bound. The hook molecule can be either attached to a solidsubstrate or the hook molecule can have a retrieval portion that can bebound to a solid substrate in a subsequent step. The solid substrate cancomprise, for example, beads, fibers, a planar substrate such as anoligonucleotide array, or column packing.

Methods for Isolating Nucleic Acid Complexes

In some aspects, the invention includes isolating polymeraseenzyme-nucleic acid complexes from other components in the reactionmixture without using walk-in to expose the capture sequence. The samehook molecules or hook oligonucleotides can be used for methods that usewalk-in and those that do not use walk-in. In some embodiments theinvention comprises a method for isolating a polymerase-nucleic acidcomplex comprising first forming a polymerase-nucleic acid complex bymixing a polymerase enzyme with a nucleic acid having a single strandedportion and a double stranded portion. A hook molecule such as anoligonucleotide is then specifically attached to the complex, e.g. byhybridization through a capture region on the hook oligonucleotide thatis targeted to the single stranded portion of the nucleic acid. Once thehook molecule is attached to the nucleic acid, the complex can beisolated using the hook oligonucleotide. For example, in some cases thehook molecule is a hook oligonucleotide that is attached to a bead. Thebead can be isolated from solution to isolate the polymerase-nucleicacid complex from the reaction mixture. The bead can be, for example, amagnetic bead.

The hook oligonucleotide used can be attached to a bead, allowing fordirect removal or isolation of the complex. In other cases, the hookoligonucleotide can have a retrieval region that is complementary to anoligonucleotide sequence attached to the bead, allowing for indirectremoval or isolation. The design of the primers is carried out tocontrol the binding strength of the various regions. For example, insome cases a bead having a poly nucleotide, such as a poly-A tail, isused, and the hook oligonucleotide has a poly(T) region and a captureregion complementary to the single stranded portion of the nucleic acid.The poly(T) region is typically designed to be stable enough for removalor isolation, but generally less stable than the binding of the captureregion to the oligonucleotide. For example, in some cases, the poly(T)region ranges from about 12 to about 30 nucleotides, or from about 15 toabout 25 nucleotides or from about 16 to about 21 nucleotides in length.The strength of the capture region is generally designed to be strongerthan the binding of the retrieval region. In some cases groups that canenhance hybridization stability such as LNAs, PNAs or methoxy groups areused. In some cases, a “splint” oligonucleotide is used which has twobinding segments, one that binds to a retrieval sequence on the hookoligonucleotide, and the other that binds to the retrieval sequence onthe bead.

The invention can be used for producing and purifying complexes from amixture of DNA fragments, for example as used in sequencing. The methodcan entail first fragmenting a double stranded DNA sample into doublestranded fragments, then ligating to each end of the double strandedfragments a hairpin to produce a population of circular DNA templateshaving a central double stranded region and hairpin regions on each end.The hairpin regions on each end are single stranded regions that can beused for both priming and capture. The population of circular DNAtemplates are exposed to a primer complementary to the single-strandedportion of a hairpin region of the template and to a DNA polymeraseenzyme having strand displacement activity. One can then add to thepopulation of complexes a hook oligonucleotide comprising a captureregion complementary to a portion of the hairpin region, and use thehook oligonucleotide to isolate the complexes to which it hybridized.This allows for isolating complexes from the mixture.

Polymerase-Nucleic Acid Complex

While many enzyme-substrate interactions are transient, some polymeraseenzymes can form relatively stable complexes with nucleic acids that canbe manipulated, purified, and then subsequently used to carry outnucleic acid synthesis. For example, DNA polymerases having relativelyhigh processivity can have strong associations with template nucleicacid molecules. An exemplary DNA Polymerase is phi-29 DNA polymerase.Methods for forming and manipulating polymerase-nucleic acid complexesare described, for example in copending U.S. Patent Application entitledPurified Extended Polymerase/Template Complex for Sequencing”61/385,376, filed Sep. 22, 2010 which is incorporated by referenceherein in its entirety for all purposes. The current invention describesways in which these complexes can be treated in order to isolate aportion of the complexes that have desired properties.

The polymerase-nucleic acid complex will have a polymerase and a nucleicacid having a double stranded region. The polymerase-nucleic acidcomplex will generally also have a primer from which a nascent nucleicacid strand will be produced complementary to a template strand of thenucleic acid. The primer is usually a short oligonucleotide that iscomplementary to a portion of the template nucleic acid. The primers ofthe invention can comprise naturally occurring RNA or DNAoligonucleotides. The primers of the invention may also be syntheticanalogs. The primers may have alternative backbones as described abovefor the nucleic acids of the invention. The primer may also have othermodifications, such as the inclusion of heteroatoms, the attachment oflabels, such as dyes, or substitution with functional groups which willstill allow for base pairing and for recognition by the enzyme. Primerscan select tighter binding primer sequences, e.g., GC-rich sequences, aswell as employ primers that include within their structure non-naturalnucleotides or nucleotide analogs, e.g., peptide nucleic acids (PNAs) orlocked nucleic acids (LNAs), that can demonstrate higher affinitypairing with the template. In some cases, the primer is added as aseparate component to form the complex; in other cases, the primer canbe part of the nucleic acid that used. For example, in some casespriming can begin at a nick or a gap in one strand of a double-strandednucleic acid.

The polymerase-nucleic acid complex has a nucleic acid that acts as thetemplate for nucleic acid synthesis. The nucleic acids that comprise thecomplex have at least a portion that is double stranded. One of the twostrands will act as the template strand for which the polymerase willproduce a complementary nascent nucleic acid strand. The other strand inthe double stranded region will be displaced by the strand displacementactivity of the polymerase enzyme. In some cases, the template strand ofthe nucleic acid will have a single stranded portion that is upstream ofthe double stranded region. Where this is the case a primer can be addedto hybridize to the single stranded portion, and nucleic acid synthesiscan proceed from the primer into the downstream double stranded region.

The template nucleic acid can be derived from any suitable natural orsynthetic source. In preferred embodiments, the template comprisesdouble stranded DNA, but in some circumstances double-stranded RNA orRNA-DNA heteroduplexes can be used. The template nucleic acid can begenomic DNA from eukaryotes, bacteria, or archaea. The template nucleicacid can be cDNA derived from any suitable source including messengerRNA. The template nucleic acid can comprise a library of double strandedsegments of DNA. The template nucleic acid can be linear or circular.For example, the nucleic acid can be topologically circular and have alinear double stranded region. A circular nucleic acid can be, forexample, a gapped plasmid. In some embodiments the nucleic acid is adouble stranded linear DNA having a gap in one of the strands. The gapprovides a site for attachment of the polymerase enzyme for nucleic acidsynthesis. The linear double stranded DNA having a double-stranded DNAadaptor can be made by ligation of DNA fragment to an adaptor throughblunt end—ligation or sticky end ligation. The ligation produces alinear DNA having a gap close to the 5′ end of one or both of thestrands. The gap can be any suitable width. For example, the gap can befrom 1 to 50 bases, from 2 to 30 bases, or from 3 to 12 bases.

The terms “nucleic acid” or “oligonucleotide” or grammatical equivalentsherein mean at least two nucleotides covalently linked together. Anucleic acid of the present invention will generally containphosphodiester bonds, although in some cases, nucleic acid analogs areincluded that may have alternate backbones, comprising, for example,phosphoramide, phosphorothioate, phosphorodithioate, and peptide nucleicacid backbones and linkages. Other analog nucleic acids include thosewith positive backbones, non-ionic backbones, and non-ribose backbones,including those described in U.S. Pat. Nos. 5,235,033 and 5,034,506. Thetemplate nucleic acid may also have other modifications, such as theinclusion of heteroatoms, the attachment of labels, such as dyes, orsubstitution with functional groups which will still allow for basepairing and for recognition by the enzyme.

The template sequence may be provided in any of a number of differentformat types depending upon the desired application. The template may beprovided as a circular or functionally circular construct that allowsredundant processing of the same nucleic acid sequence by the synthesiscomplex. Use of such circular constructs has been described in, e.g.,U.S. Pat. No. 7,315,019 and U.S. patent application Ser. No. 12/220,674,filed Jul. 25, 2008. Alternate functional circular constructs are alsodescribed in U.S. patent application Ser. No. 12/383,855, filed Mar. 27,2009, and U.S. patent application Ser. No. 12/413,258 filed Mar. 27,2009, the full disclosures of each of which are incorporated herein byreference in their entirety for all purposes.

Briefly, such alternate constructs include template sequences thatpossess a central double stranded portion that is linked at each end byan appropriate linking oligonucleotide, such as a hairpin loop segment.Such structures not only provide the ability to repeatedly replicate asingle molecule (and thus sequence that molecule), but also provide foradditional redundancy by replicating both the sense and antisenseportions of the double stranded portion. In the context of sequencingapplications, such redundant sequencing provides great advantages interms of sequence accuracy.

The nucleic acids can comprise a population of nucleic acids havinguniversal sequence regions that are common to all of the nucleic acidsin the population and also have specific regions that are different inthe different members of the population. The current invention allowsfor capturing and isolating polymerase-nucleic acid complexes usingeither the universal or the specific regions.

While in many cases nucleic acid synthesis is describe herein asextending from a primer, it is to be understood that some polymerases donot require an added external primer, and can be initiated usingterminal protein. Polymerases that can be initiated using terminalprotein include phi-29 polymerase.

Use of Active Polymerase to Release the Hook Molecule

An alternative implementation of the invention allows for using the hookoligonucleotide to select for inactive rather than active complexes. Forexample, the hairpin region of a circular template molecule havinghairpin regions on each end can have two different sites on it; one thatis complementary to a primer, and downstream of that site, a site thatwill be captured by a hook oligonucleotide. The template DNA, which canbe a library of fragments is treated with both the prime and the hookoligonucleotide. The hook oligonucleotide is complementary at its 5′end, and has a retrieval segment that is not complementary at its 3′end. Polymerase is then added, and the polymerization is carried outsuch that the DNA synthesis continues for some number of bases. Thecomplexes that are active will displace the hook oligonucleotide, butthe complexes that are not active will still have the hookoligonucleotide hybridized to them. The hook oligonucleotides can thenbe isolated or removed from solution using their retrieval region, forexample, with beads such as magnetic beads. Alternatively, the retrievalregion of the hook oligonucleotides can comprise a bead.

Polymerase Enzymes

Polymerase enzymes useful in the invention include polymerases mutatedto have desirable properties for sequencing. For example, suitableenzymes include those taught in, e.g., WO 2007/076057 POLYMERASES FORNUCLEOTIDE ANALOGUE INCORPORATION by Hanzel et al., WO 2008/051530POLYMERASE ENZYMES AND REAGENTS FOR ENHANCED NUCLEIC ACID SEQUENCING byRank et al., and U.S. patent application Ser. No. 12/584,481 filed Sep.4, 2009, by Pranav Patel et al. entitled “ENGINEERING POLYMERASES ANDREACTION CONDITIONS FOR MODIFIED INCORPORATION PROPERTIES.” The modifiedpolymerases may have modified properties such as e.g., decreased branchfraction formation, improved specificity, improved processivity, alteredrates, improved retention time, improved stability of the closedcomplex, etc.

In addition, the polymerases can be further modified forapplication-specific reasons, such as to increase photostability, e.g.,as taught in U.S. patent application Ser. No. 12/384,110 filed Mar. 30,2009, by Keith Bjornson et al. entitled “Enzymes Resistant toPhotodamage,” to improve activity of the enzyme when bound to a surface,as taught, e.g., in WO 2007/075987 ACTIVE SURFACE COUPLED POLYMERASES byHanzel et al. and WO 2007/076057 PROTEIN ENGINEERING STRATEGIES TOOPTIMIZE ACTIVITY OF SURFACE ATTACHED PROTEINS by Hanzel et al., or toinclude purification or handling tags as is taught in the citedreferences and as is common in the art. Similarly, the modifiedpolymerases described herein can be employed in combination with otherstrategies to improve polymerase performance, for example, reactionconditions for controlling polymerase rate constants such as taught inU.S. patent application Ser. No. 12/414,191 filed Mar. 30, 2009, andentitled “Two slow-step polymerase enzyme systems and methods,”incorporated herein by reference in its entirety for all purposes.

The polymerase enzymes used in the invention will generally havestrand-displacement activity. Many polymerases have this capability, andit is useful in the context of the current invention for opening up andexposing the regions of a nucleic acid sample for capture by a hookmolecule. In some cases, strand displacement is part of the polymeraseenzyme itself. In other cases, other cofactors or co-enzymes can beadded to provide the strand displacement capability.

DNA Polymerases

DNA polymerases are sometimes classified into six main groups based uponvarious phylogenetic relationships, e.g., with E. coli Pol I (class A),E. coli Pol II (class B), E. coli Pol III (class C), Euryarchaeotic PolII (class D), human Pol beta (class X), and E. coli UmuC/DinB andeukaryotic RAD30/xeroderma pigmentosum variant (class Y). For a reviewof recent nomenclature, see, e.g., Burgers et al. (2001) “Eukaryotic DNApolymerases: proposal for a revised nomenclature” J Biol Chem.276(47):43487-90. For a review of polymerases, see, e.g., Hübscher etal. (2002) “Eukaryotic DNA Polymerases” Annual Review of BiochemistryVol. 71: 133-163; Alba (2001) “Protein Family Review: Replicative DNAPolymerases” Genome Biology 2(1):reviews 3002.1-3002.4; and Steitz(1999) “DNA polymerases: structural diversity and common mechanisms” JBiol Chem 274:17395-17398. The basic mechanisms of action for manypolymerases have been determined. The sequences of literally hundreds ofpolymerases are publicly available, and the crystal structures for manyof these have been determined, or can be inferred based upon similarityto solved crystal structures of homologous polymerases. For example, thecrystal structure of Φ29, a preferred type of parental enzyme to bemodified according to the invention, is available.

In addition to wild-type polymerases, chimeric polymerases made from amosaic of different sources can be used. For example, 129 polymerasesmade by taking sequences from more than one parental polymerase intoaccount can be used as a starting point for mutation to produce thepolymerases of the invention. Chimeras can be produced, e.g., usingconsideration of similarity regions between the polymerases to defineconsensus sequences that are used in the chimera, or using geneshuffling technologies in which multiple Φ29-related polymerases arerandomly or semi-randomly shuffled via available gene shufflingtechniques (e.g., via “family gene shuffling”; see Crameri et al. (1998)“DNA shuffling of a family of genes from diverse species acceleratesdirected evolution” Nature 391:288-291; Clackson et al. (1991) “Makingantibody fragments using phage display libraries” Nature 352:624-628;Gibbs et al. (2001) “Degenerate oligonucleotide gene shuffling (DOGS): amethod for enhancing the frequency of recombination with familyshuffling” Gene 271:13-20; and Hiraga and Arnold (2003) “General methodfor sequence-independent site-directed chimeragenesis: J. Mol. Biol.330:287-296). In these methods, the recombination points can bepredetermined such that the gene fragments assemble in the correctorder. However, the combinations, e.g., chimeras, can be formed atrandom. For example, using methods described in Clarkson et al., fivegene chimeras, e.g., comprising segments of a Phi29 polymerase, a PZApolymerase, an M2 polymerase, a B103 polymerase, and a GA-1 polymerase,can be generated. Appropriate mutations to improve branching fraction,increase closed complex stability, or alter reaction rate constants canbe introduced into the chimeras.

Available DNA polymerase enzymes have also been modified in any of avariety of ways, e.g., to reduce or eliminate exonuclease activities(many native DNA polymerases have a proof-reading exonuclease functionthat interferes with, e.g., sequencing applications), to simplifyproduction by making protease digested enzyme fragments such as theKlenow fragment recombinant, etc. As noted, polymerases have also beenmodified to confer improvements in specificity, processivity, andimproved retention time of labeled nucleotides inpolymerase-DNA-nucleotide complexes (e.g., WO 2007/076057 POLYMERASESFOR NUCLEOTIDE ANALOGUE INCORPORATION by Hanzel et al. and WO2008/051530 POLYMERASE ENZYMES AND REAGENTS FOR ENHANCED NUCLEIC ACIDSEQUENCING by Rank et al.), to alter branch fraction and translocation(e.g., U.S. patent application Ser. No. 12/584,481 filed Sep. 4, 2009,by Pranav Patel et al. entitled “ENGINEERING POLYMERASES AND REACTIONCONDITIONS FOR MODIFIED INCORPORATION PROPERTIES”), to increasephotostability (e.g., U.S. patent application Ser. No. 12/384,110 filedMar. 30, 2009, by Keith Bjornson et al. entitled “Enzymes Resistant toPhotodamage”), and to improve surface-immobilized enzyme activities(e.g., WO 2007/075987 ACTIVE SURFACE COUPLED POLYMERASES by Hanzel etal. and WO 2007/076057 PROTEIN ENGINEERING STRATEGIES TO OPTIMIZEACTIVITY OF SURFACE ATTACHED PROTEINS by Hanzel et al.). Any of theseavailable polymerases can be modified in accordance with the inventionto decrease branching fraction formation, improve stability of theclosed polymerase-DNA complex, and/or alter reaction rate constants.

Many such polymerases that are suitable for modification are available,e.g., for use in sequencing, labeling and amplification technologies.For example, human DNA Polymerase Beta is available from R&D systems.DNA polymerase I is available from Epicenter, GE Health Care,Invitrogen, New England Biolabs, Promega, Roche Applied Science, SigmaAldrich and many others. The Klenow fragment of DNA Polymerase I isavailable in both recombinant and protease digested versions, from,e.g., Ambion, Chimerx, eEnzyme LLC, GE Health Care, Invitrogen, NewEngland Biolabs, Promega, Roche Applied Science, Sigma Aldrich and manyothers. 129 DNA polymerase is available from e.g., Epicentre. Poly Apolymerase, reverse transcriptase, Sequenase, SP6 DNA polymerase, T4 DNApolymerase, T7 DNA polymerase, and a variety of thermostable DNApolymerases (Taq, hot start, titanium Taq, etc.) are available from avariety of these and other sources. Recent commercial DNA polymerasesinclude Phusion™ High-Fidelity DNA Polymerase, available from NewEngland Biolabs; GoTaq® Flexi DNA Polymerase, available from Promega;RepliPHI™ Φ29 DNA Polymerase, available from Epicentre Biotechnologies;PfuUltra™ Hotstart DNA Polymerase, available from Stratagene; KOD HiFiDNA Polymerase, available from Novagen; and many others.Biocompare(dot)com provides comparisons of many different commerciallyavailable polymerases.

DNA polymerases that are preferred substrates for mutation to decreasebranching fraction, increase closed complex stability, or alter reactionrate constants include Taq polymerases, exonuclease deficient Taqpolymerases, E. coli DNA Polymerase 1, Klenow fragment, reversetranscriptases, Φ29-related polymerases including wild type Φ29polymerase and derivatives of such polymerases such as exonucleasedeficient forms, T7 DNA polymerase, T5 DNA polymerase, an RB69polymerase, etc.

In one aspect, the polymerase that is modified is a Φ29-type DNApolymerase. For example, the modified recombinant DNA polymerase can behomologous to a wild-type or exonuclease deficient Φ29 DNA polymerase,e.g., as described in U.S. Pat. Nos. 5,001,050, 5,198,543, or 5,576,204.Alternately, the modified recombinant DNA polymerase can be homologousto other Φ29-type DNA polymerases, such as B103, GA-1, PZA, Φ15, BS32,M2Y, Nf, G1, Cp-1, PRD1, PZE, SFS, Cp-5, Cp-7, PR4, PR5, PR722, L17,Φ21, or the like. For nomenclature, see also, Meijer et al. (2001) “Φ29Family of Phages” Microbiology and Molecular Biology Reviews,65(2):261-287. Suitable polymerases are described, for example, in U.S.patent application Ser. No. 12/924,701, filed Sep. 30, 2010; and Ser.No. 12/384,112, filed Mar. 30, 2009.

RNA Polymerases

In some embodiments, the polymerase enzyme that is used for sequencingis an RNA polymerase. Any suitable RNA polymerase (RNAP) can be usedincluding RNA polymerases from bacteria, eukaryotes, viruses, or archea.Suitable RNA polymerases include RNA PoI I, RNA PoI II, RNA PoI III, RNAPoI IV, RNA PoI V, T7 RNA polymerase, T3 RNA polymerase or SP6 RNApolymerase. The use of RNA polymerases allows for the direct sequencingof messenger RNA, transfer RNA, non-coding RNA, ribosomal RNA, micro RNAor catalytic RNA. Where RNA polymerases are used, the polymerizingreagents will generally include NTPs or their analogs rather than thedNTPs used for DNA synthesis. In addition, RNA polymerases can be usedwith specific cofactors. There are many proteins that can bind to RNAPand modify its behavior. For instance, GreA and GreB from E. coli and inmost other prokaryotes can enhance the ability of RNAP to cleave the RNAtemplate near the growing end of the chain. This cleavage can rescue astalled polymerase molecule, and is likely involved in proofreading theoccasional mistakes made by RNAP. A separate cofactor, Mfd, is involvedin transcription-coupled repair, the process in which RNAP recognizesdamaged bases in the DNA template and recruits enzymes to restore theDNA. Other cofactors are known to play regulatory roles; i.e., they helpRNAP choose whether or not to express certain genes. RNA dependent RNApolymerases (RNA replicases) may also be used including viral RNApolymerases: e.g. polioviral 3Dpol, vesicular stomatitis virus L, andhepatitis C virus NS5b protein; and eukaryotic RNA replicases which areknown to amplify microRNAs and small temporal RNAs and producedouble-stranded RNA using small interfering RNAs as primers.

Reverse Transcriptases

The polymerase enzyme used in the methods or compositions of theinvention includes RNA dependent DNA polymerases or reversetranscriptases. Suitable reverse transcriptase enzymes include HIV-1,M-MLV, AMV, and Telomere Reverse Transcriptase. Reverse transcriptasesalso allow for the direct sequencing of RNA substrates such as messengerRNA, transfer RNA, non-coding RNA, ribosomal RNA, micro RNA or catalyticRNA.

Thus, any suitable polymerase enzyme can be used in the systems andmethods of the invention. Suitable polymerases include DNA dependent DNApolymerases, DNA dependent RNA polymerases, RNA dependent DNApolymerases (reverse transcriptases), and RNA dependent RNA polymerases.

Initiation of Synthesis/Halting Synthesis

In order to obtain the polymerase-nucleic acid complex comprising theexposed portion for capture, it is generally required that theinitiation and the halting of the polymerase reaction be controlled. Forexample, the formation of the nucleic acid-polymerase-nucleic acidcomplex can be carried out in a medium having ionic metals that preventor inhibit synthesis. For example, Sr and or Ca can be present inconcentrations in which the polymerase is either inactive or only barelyactive. The level of catalytic metals such as Mg and Mn can also be keptrelatively low to minimize the amount of nucleic acid synthesis. Whileinitiation can also be controlled by manipulating the amount and type ofnucleotide or nucleotide analog, we have found that the complex isgenerally more stable in the presence of nucleotides or nucleotideanalogs. Other conditions such as the temperature and the pH can be usedto minimize polymerization during complex formation.

Once the complex is formed, polymerization can be initiated. Initiationcan be done by any suitable method. In some cases, the polymerasereaction can be initiated simply by adding the appropriate reagents fornucleic acid synthesis at the appropriate temperature and pH. Othersuitable methods, such as raising the temperature, for example, toinitiate synthesis by a hot-start enzyme can be used.

In order to provide an exposed region in an intact complex, it isgenerally required that the polymerase reaction be halted in acontrolled fashion. It is generally desired that the polymerase onlyproceed to past the point where the sequence to be captured is exposed,and then stop. It is also desired that the method of halting thepolymerase keep the polymerase-nucleic acid complex intact so that itcan be used for the subsequent steps of the method.

One method of stopping the enzyme in a controllable fashion is to carryout polymerization for a specific period of time under controlledconditions, at which point the enzyme synthesis activity is halted. Thecontrolled conditions will usually involve controlling reactionconditions such that the polymerase performs synthesis more slowly thanit is capable of. Slowing and controlling the enzyme can be done, forexample, by adding a non-catalytic metal such as Ca. In some cases, onlyCa is added as a divalent metal. We have found that nucleic acidsynthesis can occur slowly without the explicit addition of catalyticmetals. In some cases, an appropriate ratio of catalytic tonon-catalytic metal divalent cation will be provided to obtain thedesired rate. The ratio of Ca to Mg or Mn can be from about 10 to about200, from about 3 to about 1000, or from about 1 to about 10000.

One method of halting the reaction is to add a reagent that causes theenzyme to stop polymerizing, but keeps the enzyme intact for furtherpolymerization. A preferred reagent for halting the polymerase is Sr. Wehave found that by adding Sr at the appropriate concentration, thepolymerase reaction can be reversibly halted. The concentration of Sr tohalt the polymerase can be, for example, from about 0.2 mM to about 20mM, from about 0.01 mM to about 100 mM, or from about 1 nM to about 0.5M.

The time between initiation and halting can be from on the order ofseconds to on the order of days. Where the reaction time is fast, on theorder of seconds, it can be more difficult to control the initiation andtermination throughout the volume of the reaction. Where the reactiontime is multiple hours, there is the disadvantage of having to wait along time. Therefore, reaction times from about 10 seconds to about 4hours, about 30 seconds to about 2 hours, or about 1 minute to about 30minutes are desirable.

A method of halting the polymerase reaction is to add reagents whichbind the catalytic metal. It is known, for example, that a chelatingagent such as EDTA can complex with the catalytic divalent cations tohalt the reaction. Chelating agents must be used with care, as if thedivalent cations are complexed too effectively, it can result in adestabilization of the polymerase-nucleic acid complex. The reaction canalso be halted by changing the conditions, such as the temperature andthe pH in a manner that halts enzyme polymerization. As with chelatingagents these halting methods must be carried out with care so as not todamage the polymerase-nucleic acid complex, e.g. by denaturing theenzyme. For example, lowering of the temperature can be used to halt thereaction reversibly either alone or in combination with other methods.

In some cases, halting can be accomplished by providing only a limitingamount of reagents for the synthesis reaction. For example, thenucleotide or nucleotide analog can be provided at an amount such thatthe reaction runs out or slows down significantly as the desired amountof walk-in is reached.

The halting of the nucleic acid reaction can also be accomplished byproviding blocking groups on the nucleic acid that is being copied bythe polymerase. The blocking groups can be used to stop the polymeraseat a specific point, providing for control of exactly how much of thenucleic acid is exposed. Generally, the blocking groups create areversible stopping or pausing point, allowing the complex to bere-initiated for nucleic acid synthesis after the purification by thehook molecule. The stopping points may comprise elements such as largephotolabile groups, strand-binding moieties, non-native bases, andothers.

One may optionally employ various means for controlling initiationand/or progression of a sequencing reaction, and such means may includethe addition of specific sequences or other moieties into the templatenucleic acid, such as binding sites, e.g., for primers or proteins.Various methods of incorporating control elements into an analyticalreaction, e.g. by integrating stop or pause points into a template, arediscussed elsewhere herein and are further described in relatedapplications, U.S. application Ser. No. 12/413,258, filed Mar. 27, 2009,and U.S. application Ser. No. 12/982,029, filed Dec. 30, 2010 which areincorporated herein by reference in their entirety for all purposes.

In certain embodiments, a reaction stop or pause point may be includedwithin the template sequence, such as a reversibly bound blocking groupat one location on the template, e.g., on the linking portion that wasnot used in priming. The stop or pause point can be incorporated intoeither a single stranded or double stranded portion of the nucleic acidtemplate. Where SMRTBell™ type template nucleic acids are used, thepause point can be either in a hairpin linker region or in thedouble-stranded central portion. In some embodiments it is useful tohave the stop or pause point within the hairpin region. FIG. 7illustrates several approaches for the inclusion of stop or pausepoints. In FIG. 7(A), a library of templates is produced byfractionating a double stranded DNA sample into a population of doublestranded segments, and hairpin loops are hybridized onto each end. Inthis embodiment, a single hairpin loop having a universal priming siteis used, such that each end of the template nucleic acid has anidentical hairpin loop. A reversible stop is included in theconstruction of the hairpin loop part way into the double strandedregion, allowing a polymerase, that initiates at the primer to proceedinto the template nucleic acid, creating a nascent strand whichdisplaces the complementary strand, exposing a sequence that can beeused for capture by the hook. Here, the stop or pause point is placeddownstream of the priming site such that the polymerase will pause whileit is copying the hairpin adaptor, and before it reaches the portion ofthe nucleic acid that is derived from the sample nucleic acid which wasfragmented. Once capture and isolation are performed, the reversiblestop or pause can be removed to allow the enzyme to continuesynthesizing the nucleic acid template. This embodiment can be usefulwhere a universal capture region on the hook is used to capture allactive complexes regardless of the sequence in the central region.

FIG. 7(B) shows an alternative embodiment in which the reversible stopis positioned up-stream from the priming site, such that the polymerasewill pass through the central section and proceed into the hairpin loopon the other side before being halted. This approach can be employedwhen a specific sequence within the fragmented double stranded portionis targeted for capture.

FIG. 7(C) shows a construction similar to that in FIG. 7(A), but inwhich the SMRTBell™ templates have a different hairpin adaptor on eachend. The hairpin adaptor region comprising the reversible stop also hasa priming region. The hairpin adaptor at the other end of the constructhas no priming site. This type of construction can be used where it isdesired that only one enzyme bind per template nucleic acid.

FIG. 7(D) is a construct that is similar to that of FIG. 7(B) exceptthat each end of the SMRTBell™ template has a different hairpin adaptor.Here, one of the hairpin adaptors has a priming region, and the otherhairpin adaptor has a reversible stop. This type of construction can beused where it is desired that only one enzyme bind per template nucleicacid. Techniques for producing SMRTBell™ type templates having differentadaptors at each end are described, for example, in U.S. patentapplication Ser. No. 12/413,258, filed Mar. 27, 2009, the disclosure ofwhich is incorporated by reference herein for all purposes.

A variety of synthesis controlling groups may be employed as stop orpause points, including, e.g., large photolabile groups coupled to thenucleobase portion of one or more bases in the single-stranded portionthat inhibit polymerase-mediated replication; strand-binding moietiesthat prevent processive synthesis; non-native nucleotides includedwithin the primer and/or template; and the like. The use ofstrand-binding moieties includes, but is not limited to, reversible,specific binding of particular proteins to recognition sequencesincorporated into the template (or primer bound thereto) for thispurpose. In certain embodiments, such control sequences may includebinding sites for transcription factors, e.g., repressor binding regionsprovided within the linking portion(s). For example, the lac repressorrecognition sequence is bound by the lac repressor protein, and thisbinding has been shown to block replication in a manner reversible byaddition of appropriate initiators, such as isophenylthiogalactoside(IPTG) or allolactose.

In some embodiments, primer recognition sequences and/or additionalcontrol sequences may also be provided for control of initiation and/orprogression of polymerization, e.g., through a hybridized probe orreversibly modified nucleotide, or the like. (See, e.g., U.S. PatentApplication No. 2008-0009007, the full disclosure of which isincorporated herein by reference in its entirety for all purposes.) Suchprobes include but are not limited to probes at which a polymeraseinitiates polymerization, probes containing various types of detectablelabels, molecular beacons, TaqMan® probes, Invader® probes (Third WaveTechnologies, Inc.), or the like, that can be used for various purposes,e.g., to provide indications of the commencement and/or progress ofsynthesis.

An engineered pause point (reversible or irreversible) can include oneor more non-native (non-natural) or fifth bases that do not pair withany of the four native nucleoside polyphosphates in the synthesisreaction, e.g., in the template and/or oligonucleotide probe(s), and/orthat exhibit a distinct kinetic signature during template-dependentsynthesis at such a base. Upon encountering such a base, the polymerasepauses until the complement to the non-natural base is added to thereaction mixture. Likewise, an engineered pause point could include a“damaged” base that causes a stop in replication until repair enzymesare added to the mixture. For example, a template having a pyrimidinedimer would cause the replication complex to pause, and addition of thephotolyase DNA repair enzyme would repair the problem location and allowreplication, and sequencing to continue. In yet further embodiments, acombination of modification enzymes could be used to engineer a set ofmodified bases on a template, e.g., a combination of glycosylases,methylases, nucleases, and the like. (Further information on sequencingtemplate nucleic acids comprising modifications, including detectingkinetic signatures of such modifications during single-moleculesequencing reactions, are provided in U.S. Patent Application Nos.61/201,551, filed Dec. 11, 2008; 61/180,350, filed May 21, 2009; andSer. No. 12/945,767, filed Nov. 12, 2010; and U.S. Patent PublicationNo. 2010/0221716, the disclosures of which are incorporated herein byreference in their entireties for all purposes.)

As noted elsewhere herein, stop or pause points can be engineered intovarious portions of the template, e.g., portions for which thenucleotide sequence is unknown (e.g., a genomic fragment) or known(e.g., an adaptor or linker ligated to the genomic fragment.) Forexample, SMRTbell™ templates are topologically closed, single-strandedmolecules having regions of internal complementarity separated byhairpin or stem-loop linkers, such that hybridization of the regions ofinternal complementarity produces a double-stranded portion within thetemplate. One or both of the linkers can comprise a stop or pause pointto modulate polymerase activity. In some embodiments, these regulatorysequences or sites cause a permanent cessation of nascent strandsynthesis, and in other embodiments the reaction can be reinitiated,e.g., by removing a blocking moiety or adding a missing reactioncomponent. Various types of pause and stop points are described belowand elsewhere herein, and it will be understood that these can be usedindependently or in combination, e.g., in the same template molecule.

In other embodiments, an abasic site is used as a synthesis blockingmoiety or pause point until addition of a non-natural “base,” such as apyrene, which has been shown to “base-pair” with an abasic site duringDNA synthesis. (See, e.g., Matray, et al. (1999) Nature 399(6737):704-8,which is incorporated herein by reference in its entirety for allpurposes.) Where a permanent termination of sequencing is desired, nonon-natural analog is added and the polymerase is permanently blocked atthe abasic site. DNA (or RNA) glycosylases create abasic sites that arequite different from the normal coding bases, A, T, G, and C (and U inRNA). A wide variety of monofunctional and bifunctional DNA glycosylasesthat have specificity for most common DNA or RNA adducts, including5-methylcytosine, are known in the art, with different glycosylasescapable of recognizing different types of modified DNA and/or RNA bases.The molecular structures of many glycosylases have been solved, andbased on structural similarity they are grouped into four superfamilies.The UDG and AAG families contain small, compact glycosylases, whereasthe MutM/Fpg and HhH-GPD families comprise larger enzymes with multipledomains. As an example, four enzymes have been identified in Arabidopsisthaliana in the plant pathway for cytosine demethylation. Additionally,other enzymes are also known to recognize 5-methyl cytosine and removethe methylated base to create an abasic site. Further, various enzymesare known to methylate cytosine in a sequence-specific manner. As such,a combination of a cytosine-methylase and an enzyme that creates anabasic site from a methylated cytosine nucleotide can be used to createone or more abasic sites in a template nucleic acid. The size of therecognition site of the methylase and the base composition of thetemplate determine how frequently methylation occurs, and therefore thenumber of abasic sites created in a given template nucleic acid,allowing the ordinary practioner to choose a methylase with arecognition site that produces a desired spacing between modifiednucleotides. For example, if the recognition site is three bases long,then on average an abasic site is expected every 64 bases; if therecognition site is four bases long, then on average an abasic site isexpected every 256 bases; if the recognition site is six bases long,then on average an abasic site is expected every 4096 bases; and soforth. Of course, templates with a higher GC content would be expectedto have more frequent abasic site formation, and templates with lower GCcontent would be expected to have less frequent abasic site formation.

Uracil-DNA glycosylases can also be used to introduce abasic sites intoa template nucleic acid comprising deoxyuridine nucleotides. Thisstrategy has the advantage of allowing the practitioner to choose thelocations of the abasic sites within a DNA template since deoxyuridinenucleotides are not generally found in DNA. Various methods of insertingdeoxyuridine nucleotides into a DNA template may be used, and differentmethods will be preferred for different applications. In certainembodiments, one or more site-specific deoxyuracils are incorporatedduring standard phosphoramidite oligonucleotide synthesis. To placeuracils at indeterminate positions in a DNA, replacing a portion of thedeoxythymidine triphosphate with deoxyuridine triphosphate will resultin an amplimer with random U sites in place of T sites after polymerasechain reaction. In other embodiments, deoxyuridine nucleotides areengineered into the template, e.g., by ligation of a synthetic linker oradaptor comprising one or more deoxyuridine nucleotides to a nucleicacid sequence to be sequenced. In certain preferred embodiments,deoxyuridine nucleotides are incorporated into the linker portions of aSMRTbell™ template.

To subsequently introduce abasic sites prior to sequencing, thedeoxyuridine nucleotide-containing template is subjected to treatmentwith uracil-DNA glycosylase, which removes the one or more uracil basesfrom the deoxyuridine nucleotides, thereby generating one or more abasicsites in the template. Alternatively, since the deoxyuridine nucleotidecan be recognized as a template base and paired with deoxyadenosineduring template-dependent nascent strand synthesis, thesynthesis-blocking abasic site can instead be introduced afterinitiation of the sequencing reaction, e.g., at a time chosen by thepractitioner. For example, the reaction can be initiated with adeoxyuridine-containing template, and uracil-DNA glycosylase cansubsequently be added to block the polymerase and halt the reactionafter the reaction has proceeded for a given time. As such, terminationof the reaction is optional rather than required.

While uracil-DNA glycosylase activity is useful for introducing abasicsites into a template as described above, this activity can beproblematic during the preparation of such templates. As such,strategies are typically implemented during preparation and manipulationof uracil-containing DNA, e.g., using molecular biology enzymes, toavoid uracil-DNA glycosylase activity, in particular, due to the E. coliUDG enzyme. Since a majority of standard molecular biology enzymes areoverexpressed and subsequently purified from an E. coli host, UDGactivity can be a contaminating activity that is often not monitored bythe enzyme manufacturer's quality control procedures. To mitigatecontaminating UDG activity, a commercially available UDG inhibitor, alsoknown as uracil glycosylase inhibitor or UGI (e.g., from New EnglandBiolabs, Ipswich, Mass.) can be included in molecular biology reactions.This is a small protein inhibitor from the B. subtilis bacteriophagePBS1 that binds reversibly to E. coli UDG to inhibit its catalyticactivity. UGI is also capable of dissociating UDG from a DNA molecule.Alternatively, UDG activity can be inhibited without exogenous proteinusing a chemical inhibitor of the enzyme, such as an oligonucleotidecontaining a 1-aza-deoxyribose base, a transition state analog for theUDG enzyme. This and other cationic nitrogenous sugars have been usedfor mechanistic studies of UDG activity and show potent inhibitionactivity. (See, e.g., Jiang et al. Biochemistry, 2002, 41 (22), pp7116-7124.)

In certain applications, UDG activity needs to be inhibited temporarily,and subsequently enabled to remove create an abasic site as describedabove. In some embodiments, a DNA purification that removes proteins isemployed, e.g., including a phenol-chloroform extraction with subsequentethanol precipitation, a silica-based column approach (e.g., QiaQuickcolumns from Qiagen and similar products), and/or a PEG/sodium chlorideprecipitation (e.g., AMPure beads from Beckman Coulter). Alternativelyor additionally, a commercially-available UDG enzyme that is notinhibited by UGI is added when abasic site formation is desired. Forexample, the A. fulgidus UDG is from a thermophilic organism and cannotbe inhibited by the same bacteriophage protein as is the E. coli UDGenzyme. In certain preferred embodiments, UDG inhibition is employedduring template preparation, and inhibition-resistant UDG activity isadded at a subsequent time to trigger the creation of abasic sites atdeoxyuridine nucleotides, e.g., immediately prior to or during anongoing reaction.

In some embodiments, one or more abasic sites are engineered into alinker or adapter sequence within a sequencing template molecule. Abasicsugar residues serve as efficient terminators of polymerization for manypolymerases, e.g., Φ29. 1′,2′-dideoxyribose is the most common synthetic“abasic site”. In other embodiments, a synthetic linker is incorporatedinto a linker or adaptor. For example, an internal spacer (e.g., Spacer3 from Biosearch Technologies, Inc.) or other carbon-based linker can beused in lieu of a sugar-base nucleotide. Similar to an abasicnucleotide, the polymerase will be blocked upon encountering thesemoieties in the template nucleic acid.

In certain embodiments, synthesis blocking moieties used for stop orpause points are nicks in the template nucleic acid. Nicking enzymes(e.g., nicking endonucleases) are known in the art and can be used tospecifically nick the template prior to or during a template-directedsequencing reaction. The use of site-specific nicking endonucleasesallows the practitioner to incorporate a recognition sequence at aparticular location within the template nucleic acid, and such nickingendonucleases are commercially available, e.g., from New EnglandBiolabs, Inc. For example, a linker or adapter can be synthesized with anicking endonuclease recognition sequence, ligated to a nucleic acidmolecule to be sequenced, and can be specifically nicked either beforeor during a subsequent sequencing reaction. Nicks can also be introducedby ligating duplex segments that lack either a terminal 3′-hydroxy(e.g., have a dideoxynucleotide at the 3′-terminus) and/or 5′-phosphategroup on one strand. The ligation results in covalent linkage of thephosphodiester backbone on one strand, but not on the other, which istherefore effectively “nicked.” In certain embodiments, a SMRTbell™template is constructed using a duplex (or “insert”) nucleic acidmolecule lacking a 5′-phosphate group at one or both termini. Uponligation of the hairpin or stem-loop adaptors at each end, nicks arecreated at one or both ligation site(s), depending on whether the duplexlacked a 5′-phosphate at one or both ends, respectively. In otherembodiments, a SMRTbell™ template is constructed using one or twostem-loop adaptors lacking a 3′-hydroxy group at the terminus (e.g.,comprising a 2′,3′-dideoxynucleotide rather than a 2′-deoxynucleotide).Upon ligation of one or two stem-loop adaptors lacking a 3′-hydroxygroup, one or two nicks are created at the ligation site(s), dependingon whether one or two adaptors lacked the 3′-hydroxy group,respectively. In both cases, a nick is created in the template nucleicacid, and a primer bound to one of the adaptors provides an initiationsite for the polymerase, which will process the template untilencountering a nick, at which point the polymerase will terminate thereaction by dissociation from the template. Regardless of how a nick iscreated, the position of a nick relative to the initiation site for thepolymerase determines how much of the template will be sequenced.

In certain embodiments using ligation-based technologies (e.g., theSOLiD™ System developed by Life Technologies), a pause site can beengineered by using an oligonucleotide that cannot participate in theligation reaction and that is complementary to a desired location on theset of identical template nucleic acids, e.g., on a bead. When theserial ligation reaction hits the position recognized by thispolynucleotide, the reaction cannot proceed and any reactions that havebecome asynchronous will “catch up.” The user can then unblock the oligo(e.g., using chemical treatment or photo-cleavage) and reinitiate thesequencing reaction.

In some cases, it may be desirable to provide endonuclease recognitionsites within the template nucleic acid. For example, inclusion of suchsites within a circular template can allow for a mechanism to releasethe template from a synthesis reaction, i.e., by linearizing it, andallowing the polymerase to run off the linear template, and/or to exposethe template to exonuclease activity, and thus terminate synthesisthrough removal or isolation of the template. Such sites couldadditionally be exploited as control sequences by providing specificbinding locations for endonucleases engineered to lack cleavageactivity, but retain sequence specific binding, and could therefore beused to block progression of the polymerase enzyme on a template nucleicacid.

In some cases, nicking sites, e.g., sites recognized by nickingendonucleases, may be included within a portion of the templatemolecule, and particularly within a double-stranded portion of thetemplate, e.g., in a double-stranded segment of a SMRT Bell™ or in thestem portion of an exogenous hairpin structure. Such nicking sitesprovide one or more breaks in one strand of a double-stranded sequenceand can thereby provide one or more priming locations for, e.g., astrand-displacing polymerase enzyme. A variety of nicking enzymes andtheir recognition sequences are known in the art, with such enzymesbeing generally commercially available, e.g., from New England Biolabs.

Another approach for controlling the amount of walk-in is to provide awalk-in sequence in the template nucleotide made up of fewer than thefour types of nucleotides, and adding only the nucleotides required forfilling out that sequence. When the polymerase reaction encounters thebase for which no corresponding nucleotide has been added, thepolymerase reaction will stop. After isolation of the active complex,the polymerase reaction can be resumed using a reaction medium havingthe required nucleotides for extension. For example, a region of thetemplate strand portion of the template nucleic acid can be made usingA, T, and G bases, then following this portion, a region will have oneor more C's. The primer extension reaction is carried out usingnucleotides T, A, and C, but without a G nucleotide. The extensionreaction proceeds until the polymerase reaches the position having the Cbases. Since no G nucleotide is present, the polymerase stops at thatposition. This strategy can be carried out using a sequence of threebases as described above, or may be carried out with two bases or evenwith a sequence of one base.

The number of nucleotides in the double stranded region used for walk incan vary from several nucleotides to thousands of nucleotides. In thecase of performing universal capture, and capturing a sequence withinthe double stranded portion of a hairpin adaptor, walk-in can be onaverage from about 5 nucleotides to about 1,000 nucleotides or fromabout 10 nucleotides to about 200 nucleotides, or in some cases between10 nucleotides and 100 nucleotides. In the case of performing specificcapture, the walk-in can be on average from about 20 to about 100,000nucleotides, from about 50 to about 10,000 nucleotides, or from about100 to about 1000 nucleotides.

The Hook Molecule

Once the polymerase has walked-in the desired distance into thedouble-stranded region and the polymerase reaction is halted, a hookmolecule is added in order to capture the desired active complexes inthe reaction mixture. The hook molecule has at least two portions orregions, a capture region and a retrieve (or retrieval) region. Thecapture region is designed to capture a specific sequence in thetemplate nucleic acid which is exposed by the action of a polymeraseenzyme. The retrieve region allows the hook molecule to be removed fromother components of the mixture along with the complex it has captured.The capture region can be directly connected to the retrieval region, orthe hook molecule can have an intermediate region connecting the captureand retrieval portions.

In preferred embodiments, the capture region comprises anoligonucleotide with a region complementary to the sequence on thetemplate nucleic acid that is exposed by the action of the polymerase.Where a capture oligonucleotide is used, the length of the captureregion can vary depending on the application. It is well known that thestrength and selectivity of binding of complementary or partlycomplementary oligonucleotides can be controlled by controlling thestringency of the medium, including the ionic strength of the solutionand the temperature. The capture region will generally be designed bothto have efficient and specific binding, and also such that the bindingis reversible, allowing for separation of the hook from the nucleotideafter isolation. In some cases the length of the capture oligonucleotideon the hook is from about 4 to about 100 nucleotides, from about 6 toabout 50 nucleotides, or from about 8 to about 25 nucleotides in length.A capture oligonucleotide can comprise non-natural nucleotide units,e.g. PNA.

The capture region can also comprise other suitable molecules thatspecifically bind to an exposed sequence on the nucleic acid. Forexample, the capture region can comprise transcription factors,histones, antibodies, nucleic acid binding proteins, and nucleic acidbinding agents, etc., that will bind to a specific sequence. See, e.g.Blackwell et al. Science 23 Nov. 1990:Vol. 250, 1149-1151 and Kadonagaet al. PNAS, 83, 5889-5893, 1986, and Ren et at. Science, 290,2306-2309, 2000. The capture region can comprise an antibody that isdesigned to attach to a specific sequence. For antibodies that recognizespecific nucleic acid sequences, see, for example LeBlanc et al.,Biochemistry, 1998, 37 (17), pp 6015-6022. In some cases, the captureregion can comprise agents that will specifically bind regions of thetemplate nucleic acid template that have modified or unnaturalnucleotide. For example, a antibodies against 5-MeC are used to enrichfor methylated DNA sequences (See, e.g. M. Weber, et al., Nat. Genet.2005, 37, 853, incorporated herein by reference in its entirety for allpurposes). In certain embodiments, the modification is an 8-oxoG lesionand/or the agent is a protein is selected from the group consisting ofhOGG1, FPG, yOGG1, AlkA, Nth, Nei, MutY, UDG, SMUG, TDG, or NEIL. Inother embodiments, the modification is a methylated base and/or theagent is a protein selected from the group consisting of MECP2, MBD1,MBD2, MBD4, and UHRF1. Specific binding is described also in U.S. patentapplication Ser. No. 12/945,767, filed Nov. 12, 2010.

In some cases, a single type of hook molecule comprising a single typeof capture region is added to a mixture of polymerase complexes. This isdone, for example, where a universal capture region, e.g. on a hairpinadaptor, used for isolating active polymerase-nucleic acid complexesfrom inactive complexes regardless of sequence. In some cases, a mixtureof types of hook molecules is used in which each type of hook moleculehas a capture region directed at a different sequence. The mixtures ofhook molecules are generally used for isolating nucleic acids havingspecific sequences from a population of nucleic acids that do notcontain such sequences. This method could be directed to pulling downall conserved sequences of genes from a genetic pathway, derived fromone organism, but targeted at a second distinct organism. Alternatively,a family of genetic homologs, orthologs and/or paralogs could betargeted for conservation testing. Alternatively, forensic DNAsequencing (e.g., for crime scene investigation) may target a handful ofunique identifying sequences in specific loci including, e.g., uniqueshort tandem repeats, which can enable the confident identification ofindividuals. The number of different hook molecules, each with adifferent capture sequence, can be from about 2 to about 100,000 ormore. In some cases mixtures have from about 5 to about 10,000 or fromabout 10 to about 1000 different capture regions. The isolation ofspecific nucleic acid sequences of interest is valuable when greaterefficiency of characterization is desired. For example, even withcurrent sequencing technologies, sequencing of whole genomes for manyindividuals can be impractical. However, by focusing on specific regionsof interest, characterization of many genomes can be made morepractical. See e.g. Teer J K, Mullikin J C. “Exome sequencing: the sweetspot before whole genomes”, Human Molecular Genetics. 2010 Oct. 15;19(R2):R145-51 and Mamanova L, Coffey A J, Scott C E, Kozarewa I, TurnerE H, Kumar A, Howard E, Shendure J, Turner D J. “Target-enrichmentstrategies for next-generation sequencing” Nature Methods. 2010February; 7(2):111-8.

In some cases, two or more hook molecules are employed where the captureregion or regions are on one strand of the double-stranded portion. Insome cases, two or more hook molecules are employed where the captureregion or regions of one or more of the hook molecules is on one strand,and another capture region or region is on the complementary strand.This second approach can be used, for example, for capture of speciessuch as 350 shown in FIG. 3 in which a SMRTBell™ type template nucleicacid is used and in which two active enzymes are bound.

In some cases in order to capture larger nucleic acid sequences, tilingstrategies can be used, whereby sets of shorter oligonucleotides areused with each member of the set targeted to a different portion of thelarger nucleic acid sequence. For example, in some cases it could bedesired to specifically target a 2 kb sequence of DNA within a librarygenerated by fragmenting genomic DNA. Any given fragment may only have aportion of the 2 kb sequence of interest, so in order to capture suchportions, hook oligos designed to bind to various different portions ofthe 2 kb sequence can be provided. For example, a tiling strategy couldbe employed in which a set of capture oligonucleotides was provided fortargeting on average, each 50 base region along the 2 kb sequence. Thiswould result in a set of about 40 hook oligonucleotides. The nucleicacid portion which is tiled for capture could be from about 100 togreater than 1000 kb long. In some cases it could be between about 1 kband about 100 kb. The average sequence for each tile can be varied asneeded for the application, and could range, for example, from about 20bases to about 500 bases. The number of capture sequences directed at anucleotide sequence can be, for example, from about 10 to about 1000, orfrom about 20 to about 200. The tiled capture sequences can be used toselectively capture and isolate desired sets of sequences. For example,in some cases, a specific exon, or a specific family of exons could betargeted for isolation. The exons of a specific organism such as humanor mouse could be targeted. In some cases, the nucleic acidscharacteristic of a specific virus, bacterium, or pathogen or a specificstrain can be targeted. In other cases nucleic acids representingvarious functional classes, e.g. those coding for kinases can betargeted for isolation. In some cases, nucleic acids of interest in aparticular biological process, such as those implicated in cancerprogression or response to drug therapies, can be targeted.

In some cases an iterative capture and retrieval process is employedwhere a first hook oligonucleotide targeting a first sequence is used toisolate active complex having the that sequence, then in a subsequentstep, a second hook oligonucleotide is used to capture a secondsequence. This results in the isolation of only molecules having boththe first and the second capture sequences. In some cases the first andsecond sequences can be on the same strand of the double strandedportion of the nucleic acid, and in some cases one sequence is on onestrand and the other sequence is on the other strand. In some cases,rather than a single first hook oligonucleotide, a set of first hookoligonucleotides to capture a set of first sequences is employed.Analogously, in some cases rather than a second oligonucleotide, asecond set of oligonucleotides is used to capture a set of secondoligonucleotides. These iterative isolation and purification methodsallow for selecting and isolating only complexes having a desired set ofsequences.

In some embodiments, the hook comprises beads that have two types ofcapture regions attached to them, a first capture region directed to afirst sequence, and a second capture region directed to a secondsequence. These beads are added to a solution with a mixture of templatenucleic acids, some having only the first or the second capturesequence, and some having both the first and the second capturesequence. The stringency of the solution is adjusted such that complexonly bound through a single interaction will be washed off, but complexbound through both the first region and the second region will remainbound to the beads. This provides a one-step method for isolatingnucleotides from the mixture that have two sequences of interest. Insome cases, the two sequences are on the same strand; in some cases, thetwo sequences are on opposite strands. While this approach is generallyused with two types of capture regions on a bead, the same approach canbe used employing beads having 3, 4, or more types of capture regionsattached to them, but the difficulty of controlling the hybridization todifferentiate the multiply bound species goes up with the number ofcapture regions.

The retrieve region of the hook molecule is provided for removal andisolation of the hook molecule and the polymerase-nucleic acid complexthat is associated with it. In some cases, the retrieve region comprisesa bead or other solid surface. In some cases, the retrieve regioncomprises a member of a binding pair which allows for removal of thehook by a bead or surface comprising the other member of the bindingpair. The binding pair for retrieval of the hook can bind byhybridization, ionic, H-bonding, VanderWaals or any combination of theseforces. In some cases, the retrieval can be done using hybridization,e.g. using specific sequences or by using polynucleotide sequences. Forexample, one member of the biding pair can comprise either poly(A),poly(dA), poly(C) or poly(dC), and the other binding member can comprisepoly(T), poly(dT), poly(G) or poly(dG). The length of the polynucleotidesequence can be chosen to provide the best binding and releaseproperties. The binding and release can be controlled, for example, bycontrolling the stringency of the solution. Non-natural and modifiedbases can also be used in order to control the binding and releaseproperties.

Binding members can comprise, e.g., biotin, digoxigenin, inosine,avidin, GST sequences, modified GST sequences, e.g., that are lesslikely to form dimers, biotin ligase recognition (BiTag) sequences, Stags, SNAP-tags, enterokinase sites, thrombin sites, antibodies orantibody domains, antibody fragments, antigens, receptors, receptordomains, receptor fragments, or combinations thereof.

The use of beads for isolation is well known in the life sciences, andany suitable bead isolation method can be used with the presentinvention. As described above, the beads can be part of the hookmolecule, or can be added in a subsequent step to bind to and retrievethe hook molecule. Beads can be useful for isolation in thatmolecules-of-interest can be attached to the beads, and the beads can bewashed to remove solution components not attached to the beads, allowingfor purification and isolation. The beads can be separated from othercomponents in the solution based on properties such as size, density, ordielectric, ionic, and magnetic properties. In preferred embodiments,the beads are magnetic. Magnetic beads can be introduced, mixed,removed, and released into solution using magnetic fields. Processesutilizing magnetic beads can also be automated. Magnetic beads aresupplied by a number of vendors including NEB, Dynal, Micromod,Turbobeads, and Spherotech. The beads can be functionalized using wellknown chemistry to provide a surface having the binding groups requiredfor binding to the hook molecule.

Solid surfaces other than beads can also be used to retrieve the hookmolecules having active polymerase-nucleic acid complexes attached. Thesolid surfaces can be planar surfaces, such as those used forhybridization microarrays, or the solid surfaces can be the packing of aseparation column.

Isolation/Purification

The polymerase-nucleic acid complex that is bound to the hook moleculeand retrieved can then be isolated and purified. Where the hook moleculeis bound to a solid surface such as a bead, planar surface, or column,fluid can be washed over the solid surface, removing components of theoriginal mixture that are not bound to the solid surface, leaving behindon the surface the attached complex. This washing can remove, forexample, inactive polymerase-nucleic acid complex, excess enzyme,unbound nucleic acids and other components. The wash fluid willgenerally contain components that assist in maintaining the stability ofthe polymerase-nucleic acid complex, e.g. by maintaining levels ofspecific ions, the required level of ionic strength, and the appropriatepH. The stringency of the medium is also controlled during the wash toensure that the polymerase-nucleic acid complex remains bound during thewash.

In forming the polymerase-nucleic acid complex, it is often desirable touse an excess of polymerase enzyme to ensure a high level of formationand to improve the rate of complex formation. This results in therebeing free polymerase enzyme in the solution of complex after formation.Thus, the removal of excess enzyme from active polymerase-nucleic acidcomplex is one of the aims of the method. One of the benefits of thepresent method over the use of polymerase-nucleic acid complex withoutisolation is that it frees the user to use a higher amount of polymeraseat the stage of forming the complex, resulting in higher yields ofpolymerase-nucleic acid complex. For example, prior to the use of thepresent method, enzyme to template ratios on the order of 3 to 1 weregenerally employed. With the use of this method we are able to use muchhigher levels of enzyme without the deleterious effects of having theexcess enzyme contaminating the complex. For example, we have found thatenzyme to template levels of 10:1 to 50:1 provide improved performance.

In addition, we have found that the quality of the isolatedpolymerase-nucleic acid complex can be improved by adding a polymerasetrap to the mixture after binding the polymerase-nucleic acid complex tothe solid surface. The polymerase trap is believed to bind with theexcess polymerase, allowing it to be removed from the complex bywashing. In preferred embodiments, the polymerase trap is heparin.Heparin is commercially available through a number of suppliers. Heparinisolated from porcine intestine can be used. Lower molecular weightversions of heparin can also be employed. Other polymerase traps includeoligo-DNA such as M13 DNA, M18 DNA, or single-stranded circular DNA.Heparin is generally added to the solution for a concentration of fromabout 0.05 mM to about 0.5 mM, or from about 0.001 mM to about 4.0 mM.

Once the active complexes are isolated and the unwanted components ofthe mixture separated, it is generally desirable to remove the hookmolecule from the active complex. As described above, where the hookmolecule has captured the complex by nucleic acid hybridization, thecomplex can be separated from the hook molecule by adjusting thestringency of the solution. For example, the complex can be released byraising the stringency of the solution, for example by lowering theionic strength or raising the temperature.

We have found that in some cases, it is desirable to use a hook moleculehaving both a capture region and a retrieval region, each of which bindby nucleic acid hybridization. By using this approach, one can controlwhich linkage, e.g. the capture linkage or the retrieval linkage ismaintained. It is well known that the melting temperature (Tm) of ahybridized portion of oligonucleotides can be adjusted, for example byincreasing the number of matched bases, by including unmatched bases, orby including non-natural bases (See, e.g. Sambrook and Russell,Molecular Cloning, a Laboratory Manual, 2001, Cold Spring Harbor Press).Thus the relative strength of linkages can be controlled by controllingthe relative Tm. The melting temperature (Tm) is not an absolute valuebut is dependent on various factors, for example on the ionic strengthof the solution. This allows for two linkages to be formed, one having ahigher Tm than another, then by controlling the stringency of thesolution, one can control whether both of the links, one of the links,or neither of the links are broken.

In a preferred embodiment, the linkage between the capture region of thehook oligonucleotide and the complex is designed to have a lower Tm thanthe linkage between the retrieval region of the hook oligonucleotide andthe solid substrate. This allows for the stringency of the solution tobe lowered in order to release the polymerase-nucleic acid complex fromthe hook molecule while the linkage between the hook molecule and thesolid substrate (e.g. bead) remains intact. The polymerase complex canthereby be cleaved from the solid substrate into solution, leaving thehook molecule behind. In some cases, the Tm of the hook-to-complexlinkage is between about 2 degrees and about 10 degrees below the Tm ofthe hook-to-solid substrate (e.g. bead) linkage, in some cases, the Tmof the hook to complex linkage is between about 5 degrees and about 50degrees below the Tm of the hook to solid substrate (e.g. bead) linkage.

As described in more detail below, we have found that one method forremoving the polymerase-nucleic acid complex from the hookoligonucleotide is to bring the complex into physical contact with asubstrate that binds to the polymerase-nucleic acid complex. For thistype of physical removal of the complex, we have found that engineeringthe same types of hybridization interactions as described above forremoval of complex by adjusting solution stringency can be used. Forexample, using the methods described above, a construct comprising apolymerase-nucleic acid complex captured by a hook oligonucleotideretrieved by a bead can be formed. The polymerase in the complex hasattached to it a binding moiety such as a biotin or streptavidin. Thebeads are then brought into proximity or into physical contact with asubstrate comprising binding groups that bind to the polymerase enzyme.The binding of the polymerase to the surface is engineered such that itis stronger than the linkage between the hook oligonucleotide and thecomplex or the hook oligonucleotide and the solid surface, resulting inthe complex remaining on the surface when the bead is moved relative tothe surface. For example, a biotin-streptavidin type linkage isgenerally stronger than a linkage formed through hybridization ofoligonucleotides on the order of 5 to 40 nucleotides, thus abiotin-streptavidin type linkage is used at the surface, and ahybridization linkage, e.g. of between about 5 and 40 matchednucleotides, is used for the other linkages. For the physical releasemethods as for the solution methods, the linkage between the complex andthe hook oligonucleotide can be produced to have a lower Tm than thelinkage between the hook oligonucleotide and the bead, resulting in ahigher likelihood of breaking the bond between the hook oligonucleotideand the complex, resulting in delivery of the complex to the surface. Insome cases, the Tm of the hook-to-complex linkage is between about 2degrees and about 10 degrees below the Tm of the hook-to-solid substrate(e.g. bead) linkage, in some cases, the Tm of the hook-to-complexlinkage is between about 5 degrees and about 50 degrees below the Tm ofthe hook-to-solid substrate (e.g. bead) linkage.

While in many cases it is desirable to selectively break the linkagebetween the hook molecule and the complex. There may also be cases whereit is preferred to selectively break the linkage between the solidsubstrate and the hook molecule. Such approaches can also be implementedas part of the invention.

In some cases, the hook oligonucleotide comprises a primer. For example,the capture portion of the hook molecule can act as a primer forpolymerase mediated nucleic acid synthesis, for example, to carry outsingle molecule sequencing. For example, where a template moleculecomprising a double stranded region and a single stranded region (e.g. ahairpin region) is used as the template, a hook molecule having acapture region and a retrieval region can be hybridized through thecapture region to the single stranded portion of the template. Thecapture region can act as a primer for the complexation of a polymeraseto form a polymerase-nucleic acid complex. This complex can then beisolated or removed from using the retrieval region of the hookmolecule. The retrieval region can either comprise a bead or cancomprise a coupling portion that can couple to the bead. For example,the retrieval region can comprise a poly(A) or poly(dA) region that canhybridize with a bead having poly(T) or poly(dT) moities attached.

Conditions for Nucleic Acid Synthesis

The conditions required for nucleic acid synthesis are well known in theart. The polymerase reaction conditions include the type andconcentration of buffer, the pH of the reaction, the temperature, thetype and concentration of salts, the presence of particular additivesthat influence the kinetics of the enzyme, and the type, concentration,and relative amounts of various cofactors, including metal cofactors.

Enzymatic reactions are often run in the presence of a buffer, which isused, in part, to control the pH of the reaction mixture. Bufferssuitable for the invention include, for example, TAPS(3-{[tris(hydroxymethyl)methyl]amino}propanesulfonic acid), Bicine(N,N-bis(2-hydroxyethyl)glycine), TRIS (tris(hydroxymethyl)methylamine),ACES (N-(2-Acetamido)-2-aminoethanesulfonic acid), Tricine(N-tris(hydroxymethyl)methylglycine), HEPES4-2-hydroxyethyl-1-piperazineethanesulfonic acid), TES(2-{[tris(hydroxymethyl)methyl]amino}ethanesulfonic acid), MOPS(3-(N-morpholino)propanesulfonic acid), PIPES(piperazine-N,N′-bis(2-ethanesulfonic acid)), and MES(2-(N-morpholino)ethanesulfonic acid).

The pH of the reaction can influence the rate of the polymerasereaction. The temperature of the reaction can be adjusted to enhance theperformance of the system. The reaction temperature may depend upon thetype of polymerase which is employed.

As used in the art, the term nucleotide refers both to the nucleosidetriphosphates that are added to a growing nucleic acid chain in thepolymerase reaction, and also to refer to the individual units of anucleic acid molecule, for example the units of DNA and RNA. Herein, theterm nucleotide is used consistently with its use in the art. Whetherthe term nucleotide refers to the substrate molecule to be added to thegrowing nucleic acid or to the units in the nucleic acid chain can bederived from the context in which the term is used.

The nucleotides or set of nucleotides used during nucleic acid synthesisare generally naturally occurring nucleotides but can also includemodified nucleotides (nucleotide analogs). The nucleotides used in theinvention, whether natural, unnatural, modified or analog, are suitablefor participation in the polymerase reaction. The term nucleotide mayalso be used to refer to nucleotides having other than three phosphategroups, for example 4, 5, 6, 7 or more phosphate groups. Suchnucleotides have been described, for example in U.S. Pat. Nos. 6,936,702and 7,041,812. Labels such as fluorescent dye groups may be located invarious positions on the nucleotide. In some cases, a fluorescent dye islocated on the terminal phosphate of the nucleotide.

The nucleotide compositions may include nucleoside triphosphates, oranalogs of such compounds. For example, in some cases, the reactionmixtures will include nucleotide analogs having longer phosphate chains,such as nucleoside tetra, penta-, hexa- or even heptaphosphates. Inaddition, the nucleotide analogs of the compositions of the inventionmay additionally include other components, such as detectable labelinggroups. Such detectable labeling groups will typically impart anoptically or electrochemically detectable property to the nucleotideanalogs being incorporated into the synthesis reaction. In particularlypreferred aspects, fluorescent labeling groups, i.e., labeling groupsthat emit light of one wavelength when excited with light of anotherwavelength, are used as the labeling groups. For purposes of the presentdisclosure, the foregoing or later discussed nucleotide or nucleotideanalog compositions whether labeled or unlabeled, possessing of one ormore phosphate groups, typically two or more or three or more phosphategroups, or otherwise modified, are generally referred to herein asnucleotides.

Physical Transfer of Isolated Polymerase-Nucleic Acid Complexes toSubstrates

We have found that the constructs of the present invention allow fordeposition of isolated polymerase-nucleic acid complexes directly ontosubstrates. We have found that this can be accomplished without the stepof removing the polymerase-nucleic acid complex from the solid surface(e.g. bead). In general, the method comprises obtaining solution ofbeads that have polymerase-nucleic acid complexes attached to them. Thesolution of beads is brought into contact with a substrate onto which itis desired to deposit the complexes. The substrate that is used isprepared to have groups that bind to the polymerase-nucleic acidcomplexes. After the solution of beads is brought into contact with thesurface, the beads are removed, leaving polymerase-nucleic acidcomplexes bound to the substrate. We have found that prior to removal ofthe beads from the substrate, it is also generally desirable to inducemovement between the beads and the substrate, e.g. moving the beadsacross the surface of the substrate in order to increase the number ofcomplexes that are deposited. We have found that the physical depositionmethods allow for depositing polymerase-nucleic acid complexes usingrelatively low amounts of material as compared to depositing fromsolution of polymerase-nucleic acid complexes. One way to interpret thisresult is that by attaching the polymerase-nucleic acid complexes to thesurfaces of the beads and bringing the beads into proximity or intocontact with the substrate, the effective concentration of the complexesat the surface is increased over what it would be if the same number ofmolecules had to reach the substrate by diffusion.

The beads coated with polymerase-nucleic acid complex can be produced asdescribed herein or in any other suitable manner. While the invention isdescribed in terms of beads, it is to be understood that other solidsurfaces having polymerase-nucleic complexes attached can be used, aslong as the solid surface can be brought into proximity or into contactwith the substrate to deposit polymerase-nucleic acid complexes. Thebeads are generally spherical, but can have any other suitable shape,for example fibers, rods, disks, cubes, or other shaped materials can beused. Beads are useful as they can be readily manipulated within asolution. Beads for use in the invention can be functionalized on theirouter surfaces for the attachment of polymerase-nucleic acid complexes.Suitable beads include polymeric beads having functional organicmolecules on their surfaces allowing for such attachment. A variety oftypes of types of beads are known and used and many are commerciallyavailable. The beads can be produced in various size ranges from thenanometer to the millimeter size range. In some cases, the beads can beproduced to be relatively monodisperse, which can be helpful inobtaining consistent results.

The beads can be brought into proximity or contact with the substrate ina variety of ways. Forces such as gravitational force, centrifugalforce, magnetic, electrical, or dielectric forces or a combinationthereof can be used to bring the beads contact the beads with thesurface and to move the beads with respect to the surface. In preferredapproaches, magnetic beads are used, and magnetic fields are appliedboth to bring the beads down into proximity or into contact with thesubstrate and to move the beads across the substrate.

Magnetic beads have been used for purification and separation inchemical and biochemical processes, and functionalized magnetic beadsare commercially available. For example, NEB offers a variety ofmagnetic beads including Amylose Magnetic Beads, Anti-MBP MagneticBeads, Chitin Magnetic Beads, Goat Anti-Mouse IgG Magnetic Beads, GoatAnti-Rabbit IgG Magnetic Beads, Goat Anti-Rat IgG Magnetic Beads,Hydrophilic Streptavidin Magnetic Beads, Protein A Magnetic Beads,Protein G Magnetic Beads, Streptavidin Magnetic Beads, SNAP-CaptureMagnetic Beads, Oligo(dT) Magnetic Beads; Dynal (Life Technologies)offers a variety of functionalized magnetic beads including streptavidincoated beads, beads for binding with His tags, anion exchange, cationexchange, hydrophobic capture, and antibody beads. Micromod offersmagnetic beads functionalized with surface functionalities NH2, PEG-NH2and PEG-COOH for the covalent binding of proteins, antibodies or othermolecules. Tubobeads LLC offers beads having streptavidin, sulfonate,carboxylate, or ammonium functionality. Spherotech Inc. offers magneticbeads having a variety of functionalities including carboxyl, amino,antibodies, and proteins. Using functionalized beads and known methodsof surface polymer synthesis, beads with a variety of properties can bemade, including those having oligonucleotides or peptides havingspecified sequences.

The beads can comprise polymers including polystyrene/polymethacrylate,dextran, crosslinked dextran, silica-fortified dextran, starch(BNF-starch particles), poly(lactic acid), poly(ethylene imine), orchitosan. The beads can also be made from inorganic material such ascarbon, iron oxide, silica, or silicon. The magnetic beads can be usefulas long as they are effectively moved by an applied magnetic field. Forexample, the beads can be ferromagnetic or paramagnetic, orsuperparamagnetic.

The methods of the invention can also be used to deposit molecules otherthan polymerase-nucleic acid complexes. The invention allows for loadingenzymes, proteins, nucleic acids, sugars, or other molecules onto asurface. In some embodiments the invention comprises providing asolution of beads, the beads having attached to them amolecule-of-interest through a bead to molecule-of-interest linkage;exposing the solution of beads to a substrate having a plurality ofbinding molecules for binding the molecule-of-interest to the substrate;bringing the beads into physical contact with the substrate using aforce; optionally providing a force to move the beads across thesubstrate surface; then removing the beads from the substrate leavingmolecules-of-interest attached to the substrate surface through thebinding groups. The bead to molecule-of-interest linkage is preferablyformed by oligonucleotide hybridization. As described herein, therelative strength of such a linkage can be engineered such that thislinkage will break more easily than the bond between themolecule-of-interest and the substrate, allowing for removal of thebeads while leaving the molecule-of-interest on the substrate. In somecases, the substrate comprises an array of nanoscale wells such as ZMWswhere binding molecules are present on the bases of the nanoscale wellssuch that molecules-of-interest are deposited into the nanoscale wells.

The molecule-of-interest that can be deposited onto a substrate can beany suitable molecule including for example, proteins, nucleic acids,sugars, or combinations thereof. The molecules-of-interest can includebiomolecules and catalysts, and enzymes.

In some embodiments the method comprises: providing a solution of beadswherein each bead comprises a single molecule-of-interest or a pluralityof molecules-of-interest linked thereto by a bead tomolecule-of-interest linkage; exposing the solution of beads to asubstrate, the surface of the substrate comprising binding molecules forbinding the molecules-of-interest; using a contacting force to bring thebeads into proximity or into physical contact with the substrate andoptionally using a distributing force to move the beads across thesurface of the substrate; and removing the beads from the substrate,thereby producing a substrate having molecules-of-interest bound to itssurface through the binding molecules.

The methods, compositions, and devices of the invention are particularlyuseful for performing single-molecule analysis. A reason for this isthat the methods are useful for providing molecules-of-interest such aspolymerase-nucleic acid complexes at relatively sparse levels on asubstrate. Thus the method can be used to deposit molecules-of-intereston a substrate such that the molecules-of-interest are provided at asurface density such that the molecules-of-interest are independentlyoptically observable. In some cases, the substrate comprises an array ofnanoscale wells such as arrays of zero mode waveguides (ZMWs). Forexample, the substrate can have a transparent lower layer comprised, forexample, of fused silica, upon which is deposited a cladding layer witha thickness of between about 10 nm and about 500 nm. The cladding layeris generally an opaque layer and can be a metal layer. Through thecladding layer is an array of holes extending to the transparentsubstrate, and in some cases extending into the transparent substrate.The holes can have any suitable cross-sectional profile including acircular profile. Where the holes have a circular profile, the diameterof the holes is generally from about 20 nm to about 500 nm. The holesextending to the transparent substrate will generally have a portion ofthe transparent substrate as their base, thus forming nanoscale wells.For use in the present invention, the arrays of nanoscale wells arefunctionalized such that binding molecules are attached at the bases ofthe wells for binding the molecule or molecules-of-interest, such as apolymerase-nucleic acid complex, within the well. In some cases, thearrays are selectively functionalized such that a higher density ofbinding molecules is present within the wells than outside of the wells.Approaches to functionalizing zero mode waveguide substrates areprovided in U.S. Pat. Nos. 7,833,398, 7,292,742 and in U.S. patentapplication Ser. No. 11/731,748, filed Mar. 29, 2007, Ser. No.12/079,922, filed Mar. 27, 2008, and Ser. No. 12/074,716, filed Mar. 5,2008, the full disclosures of which are incorporated by reference hereinfor all purposes. As described elsewhere herein, these nanoscale wellsprovide for carrying out analyses on very small numbers of moleculesdown to single molecules. In some cases the methods, devices, andcompositions of the invention allow for the deposition of singlemolecules-of-interest within nanoscale wells.

When depositing molecules-of-interest, e.g. polymerase-nucleic acidcomplexes into ZMWs, in some cases, we have found that it is desirablefor the diameter of the beads to be larger than the smallestcross-sectional dimension for the ZMW; where the ZMW has a circularprofile, larger than the diameter of the ZMW. In some cases the diameterof the bead is 20% greater or more than the smallest cross-sectionaldimension of the ZMW. In some cases the diameter of the bead is 2 timesgreater or more than the smallest cross-sectional dimension of the ZMW.In some cases the diameter of the bead is 2 times greater to 10,000times greater than the smallest cross-sectional dimension of the ZMW. Inother cases, it can be useful to have the size of the bead be smallerthan the size of the ZMW. The size of the beads can be, for example,from about 40 nm to about 10 microns in diameter.

As would be understood in the art, the beads generally do not have aperfectly spherical shape, and are generally not perfectly monodisperse,but will have a distribution of sizes and shapes. In addition, where theoutsides surfaces of the particles are composed of polymers that aresoluble or partly soluble in the solution, the surfaces are not smoothflat surfaces, but the groups attached to the surface can extend fromthe bead on polymer chains into the solution. Though not bound bytheory, it is believed that in some cases these polymer chains extendinginto solution can provide polymer-nucleic acid complex into nanoscalewells from beads that would be too large to fit into the wells. Thisproperty can be used to advantage in the loading of ZMWs. In some cases,spacer or linker molecules are provided on the bead surface between afunctional group on the bead and the group that is used to link to thehook molecule or to link directly to the molecule-of-interest such asthe polymerase-nucleic acid complex. By varying the length of the spaceror linker, one can provide for more or less reach between the surface ofthe bead and the molecule-of-interest. The spacer or linker can be anysuitable molecular structure. It can be made, for example from a polymersuch as polypeptide, poly(vinyl alcohol), poly ethylene glycol, orpolysaccharide. The linker will generally be made using a polymer thatis soluble in the solution that the bead deposition takes place in.Where the molecule-of-interest is an enzyme, this is generally a polarsolution, such as an aqueous environment, for which a polar orhydrophilic linker or spacer is used.

In some aspects, the invention provides a method for loading activepolymerase-nucleic acid complexes onto a substrate comprising: providinga solution of magnetic beads having polymerase-nucleic acid complexesbound thereto, each polymerase-nucleic acid complex comprising apolymerase enzyme and a template nucleotide; contacting the solution ofmagnetic beads with the top of a substrate comprising an array ofnanoscale wells having bases, wherein the bases of the wells havecoupling agent bound thereto; applying a dynamic magnetic field frombelow the substrate to move the magnetic beads in solution down to thetop of the substrate, whereby the dynamic magnetic field causes theparticles to be moved across the top surface of the substrate, wherebysome polymerase-nucleic acid complexes become bound to the couplinggroups on the bases of the nanoscale wells. In some cases, the magneticfield is applied from above or adjacent to the substrate. For example,field focusing can be used which allows for applying magnetic fieldsfrom above, yet obtaining a field in which the field gradient is highestbelow the substrate, tending to pull the magnetic beads down.

The coupling groups or binding molecules on the substrate for couplingto the molecule-of-interest, e.g. polymerase-nucleic acid complex, canbe any suitable coupling group or binding molecules. The coupling can beaccomplished by forming a covalent bond or through a non-covalentinteraction. It is generally desired that the coupling to the substrateresult in a strong bond relative to the other linkages, e.g. between thepolymerase-nucleic acid complex and the hook molecule and between thehook molecule and the bead. Many types of binding pairs are known in theart. In some cases, an interaction between biotin and a biotin bindingprotein such as avidin or streptavidin is used. In some cases, anantibody-antigen interaction, for example between digoxigenin andanti-digoxigenin is used. Reactions that form covalent linkages, forexample SNAP or Click chemistry can be used to bind thepolymerase-nucleic acid complex to the substrate. Oligonucleotidehybridization can also be used for the attachment. Where suchhybridization is used, the linkages are designed such that theoligonucleotide binding to the surface is stronger, e.g. has a higherTm, than the other linkages between the surface and the bead.

Binding of the polymerase-nucleic acid complex to the substrate isgenerally carried out by forming a bond to the polymerase. One member ofthe binding pair is generally used to attach the complex to thesubstrate is connected directly or indirectly to the polymerase. In somecases, a biotinylation sequence is included when producing thepolymerase, the protein is biotinylated and attached to streptavidinprior to formation of the complex. The polymerase-streptavidin is thenready for binding to a substrate that is prepared by having biotingroups on its surface.

Where the molecule-of-interest comprises a polymerase-nucleic acidcomplex, the solution that is used for deposition with beads isgenerally an aqueous solution. The components of the solution and theconditions are controlled as described above in order that thepolymerase-nucleic acid complex remains intact. For example, theappropriate level of monovalent and divalent ions, the concentration ofnucleotide, the pH and the temperature are controlled. It is alsogenerally desired that the polymerase not continue to perform nucleicacid synthesis during deposition, and Sr and Ca can be added in order toinhibit or reduce polymerization.

There is generally a plurality of molecules-of-interest attached to abead. For example, there can be from tens to millions or more ofmolecules attached to a bead. In some cases, the beads, or a subset ofthe beads will each only have one molecule-of-interest attached.

Where beads are used to selectively deliver molecules-of-interest to thesubstrate, the beads can be brought into contact with the substrate byapplying a force to the beads which can involve placing the beads in afield which applies such a force. We have found that an effectiveprocess for binding the molecules-of-interest generally involvesapplying both a field that forces the beads down to the surface of thesubstrate and a field that moves the beads across the surface of thesubstrate. These two fields can be different fields, or can be twocomponents of the same field. The fields can be, for example,gravitational, centrifugal, magnetic, electric, or dielectric.

Preferred embodiments of the invention utilize a magnetic field both tobring down the particles and to move the particles across the surface ofthe substrate, either in contact with or in proximity to the substrate.The magnetic field can be applied using one or more permanent magnets,or using one or more electromagnets. Each of these approaches has itsbenefits and drawbacks, and each can be employed to carry out theinvention. In some cases, one, two, three, four or more permanentmagnets are held below the substrate, and are continuously moved withrespect to the substrate. In this manner, the beads are both pulled downto the substrate and are moved across the substrate surface. Themovement of the magnet or magnets can be in any pattern that providessuitable movement of the beads. The beads can be moved around in theplane of the substrate, or can be moved such that they move away fromand back toward the substrate as well. A circular movement of one ormore magnets underneath the substrate has been found to bestraightforward to implement and to provide the requisite movement. Insome cases, the magnets can remain fixed and the substrate moved withrespect to the magnets. In some cases, both the substrate and the magnetare moved.

The choice of the mode of magnetic movement will also depend on the sizeand shape of the substrate to which the beads are to be contacted ormoved into proximity of. For example, the magnets can be made to tracewider circles to ensure that the beads come into contact with the outerregions of a larger surface. In some embodiments, two magnets held nextto one another under the substrate are used, one having its north polefacing upward, and the other having its north pole facing downward. Thispair of magnets is attached to a mechanism that rotates the pairunderneath the substrate. The pair of magnets is rotated in the plane ofthe substrate below the substrate at about 10 to about 120 rpm. In somecases, rotation rates of 1 rpm to 600 rpm, 3 rpm to 120 rpm, or 6 rpm to20 rpm are used. The beads are moved across the substrate typically forabout 5 to about 20 minutes, but in some cases for about 1 minute toabout 2 hours. A variety of permanent magnets are readily availablecommercially. For example, Dura Magnetics Inc. has available on theirwebsite (http://www.duramag.com/magnet-materials.html) various magnetsincluding magnets having various magnetic strengths. The type and shapeof the permanent magnet can be chosen for ease of implementation and tooptimize loading. For example, button magnets, bar magnets, or sheetmagnets can be employed.

One or more electromagnets can also be utilized to move the particlesfor deposition. For example one or more electromagnets can be mountedbelow the substrate, and the current to the electromagnet(s) can bevaried in order to vary the strength of the magnetic field. By placingmultiple electromagnets in a pattern, and controlling the current toeach of the electromagnets, a moving magnetic field can be producedabove the substrate which can both bring down the magnetic particles andmove the particles across the substrate surface. The use ofelectromagnets has the advantage that a system for moving the beads canbe constructed with no moving parts. The current flowing through theelectromagnets will produce heat at the electromagnet. When using thisapproach, this heat generation should be taken into account. In somecases, when using electromagnets, heat-sinking, insulation, and/oractive cooling is provided to control the temperature.

The magnetic strength, number of magnets, speed of movement, distancefrom substrate, and time of deposition can be varied to obtain thedesired results. We have found that even for very small magnetic beads,microscopy can be used to observe the behavior of the cloud of beadsbeing moved by the magnetic field in real time. These observations canalso be used to set the appropriate parameters for deposition.

Gravitational fields can be used for relatively large beads. As thebeads get smaller, the ability of a gravitational force to move thebeads down from solution becomes limited. We have found that beadshaving a mean diameter of about 3 microns can be deposited onto a zeromode waveguide chip to load polymerase-nucleic acid complexes usinggravity alone. Loading was seen with exposures less than 1 hour. Wefound that gravitational loading will result in higher levels of loadingfor larger templates just as was seen with magnetic loading. In somecases, the chip can be slowly rotated while the beads move across thetop of the surface. The rotation allows for the beads to move relativeto the chip surface. In some cases, the chip is tilted while it isrotated to facilitate the movement of the beads across the surface.Centrifugal fields can also be applied to bring down the beads and alsoto move the beads across the surface of the substrate. For example, thesubstrates can be mounted within a centrifuge such that the substrate isat an angle with the centrifugal force vector, and the substrate can berotated such that the beads move around across its surface.

Electric fields can be used to move the particles where the particleshave the characteristics that they will move in an electric field. Forexample, particles having a net charge, or particles made of a polymerhaving a net charge surrounded by counterions of the opposite charge,will move in an electric field. As with the description above for themagnetic field, a dynamic electric field can be used to both move theparticles to the substrate and to move the particles across the surfaceof the substrate. Typically electrodes will be placed in contact withthe solution. The appropriate voltages are then applied to theelectrodes as a function of time to produce the electric field.Particles can also be made to move according to the invention usingdielectric field gradients and alternating currents. Acoustic fields(sonication) can be used to move the beads relative to the surface.Hydrodynamic forces, e.g. through creation of a vortex, can also beutilized.

Combinations of fields can also be used. For example a magnet can beused to pull down the beads and another force, such as ultrasonication,can be used to move them, or centrifugation can be used to pull down thebeads and a separate force used to move them.

One object of the invention is providing molecules-of-interest such aspolymerase-nucleic acid complexes to a substrate for single moleculeanalysis. For single molecule analysis it is generally desired thatsingle molecules-of-interest are bound to a substrate at a density andpattern such that the optical signal from one molecule can be detecteddistinctly from signals from other molecules and from solution. That is,the molecules are deposited so as to be individually opticallyresolvable. One method that has been used for this purpose is to depositmolecules-of-interest from a solution that is diluted such that onaverage, an acceptable number of single molecules will be individuallyoptically resolvable. If the concentration is too high, the density onthe surface will be such that few, if any, single molecules will beresolvable. If the concentration is too low, this may also result invery few single molecules. The methods, devices and compositions of thepresent invention provide an alternative approach for obtaining highlevels of optically resolvable single molecules on a substrate.

As described above, a preferred substrate for single-molecule analysisis a zero mode waveguide (ZMW) array. Here, the optical analysis iscarried out only within the ZMWs on the surface. We have found that theinvention provides useful methods for loading single molecules into aZMW array. As with other substrates for single molecule analysis,loading molecules-of-interest onto ZMWs to obtain acceptable numbers ofsingle molecules has often been carried out with a dilution method wheresolutions at various dilution levels are applied to the surface toobtain the optimal loading. The methods of the invention provide toolsfor controlling the way in which molecules-of-interest are loaded intoZMWs.

When depositing a library of polymerase-nucleic acid complexes onto asubstrate, for example a ZMW substrate, by diffusion from solution, wehave found that there can be a relatively large number of smallerfragments deposited than of larger fragments. We have found that bydepositing with beads, there can be a much more even distribution ofdeposited polymerase-nucleic acid complexes by size, allowing for abetter representation of the larger size fragments in the data in singlemolecule analysis. In some cases, bead loading also allows forpreferential loading of larger size fragments over smaller sizefragments.

Since ZMWs are wells with defined dimensions, the sizes, shapes, andextension (reach) of the beads can be used to control the manner inwhich molecules-of-interest are deposited. For example in some cases,beads are used that have a size that is smaller than a characteristicdimension of the ZMW, such that a bead fits into a ZMW, and has a reachsuch that only molecules-of-interest from a bead fitting into the ZMWwill be deposited. In some cases, beads will be used that are smallerthan the diameter of a ZMW, but larger than half of the diameter of theZMW. In this way, only one bead will deposit into the ZMW, preventingthe deposition of a second bead, ensuring that each ZMW will onlyreceive molecules-of-interest from one bead. For example, for a ZMWarray having ZMWs with diameters of 200 nm, beads having diameters fromabout 100 nm to about 190 nm are used. Another way of controlling thelevel of loading is by controlling the density of molecules-of-intereston the surfaces of the beads. For example, by using sparselyfunctionalized beads, only small numbers of molecules-of-interest willbe deposited.

When loading a surface for single molecule analysis, generally a smallamount of material is deposited as compared to the total amount on thebead. This allows for re-using the beads by removing them from thesubstrate, optionally storing them, and then applying them to anothersubstrate. The beads can be re-used in some case to load substrates 1,2, 3, 4, 5, 10, 20 or more times while still obtaining acceptableloading. We have found that after each loading, the amount loaded ontothe next substrate may be slightly less, but that the levels on thelater substrates are still acceptable. Comparable levels can also beobtained on later substrates by changing deposition conditions, forexample by lengthening the time of deposition. The ability to re-use thebeads can be important for getting the most out of small samples. Theability to store the beads for future loading and testing can beimportant for the integrity of the data from a study. We have also foundthat the beads with bound polymerase-nucleic acid complex can be storedfor days, weeks, and for over a month without any measurabledeterioration in properties.

Physical Transfer of Biological Molecules to Substrates

The devices, systems, and methods described herein for transferringtemplate-polymerase complexes onto substrates can also be used totransfer other biomolecules onto substrates and into zero modewaveguides. The methods comprise, for example, attaching a biomoleculeor a population of biomolecules to beads, and drawing the beads down toa surface to transfer the biomolecules to the surface. The biomoleculesare preferably attached to the beads by association or hybridizationsuch that the attachment can be broken to leave the biomolecule attachedto the surface even if the bead is removed from the area. The beads canbe magnetic beads that are drawn to the surface and optionallytranslated with respect to the surface during the loading process asdescribed herein. The surface will generally have reactive componentsthat will react with the biomolecule or with a molecule associated withthe biomolecule to attach it to the surface. in some cases, thesubstrate comprises an array of zero mode waveguides functionalized onthe bases of the zero mode waveguides to provide attachment of thebiological molecules within the observation region of the zero modewaveguide.

The biological molecules can be any suitable biomolecule including aprotein, a nucleic acid, a lipid, a polysaccharide, or a combination ofthese types of molecules. In some embodiments enzymes are loaded ontothe substrate. Many types of enzymes are known in the art that can beused herein. The biological molecules can be comprise constructs made ofassociated subunits that are bound onto the surface together. Forexample, the biological molecules can comprise a ribosome. Thebiological molecules can comprise antibodies or binding proteins.

Sequencing by Incorporation

The methods, devices, and compositions of the invention are particularlyuseful for single molecule sequencing, and specifically single moleculesequencing by incorporation in real time. For sequencing processes thatrely upon monitoring of the incorporation of nucleotides into growingnascent strands being synthesized by the complex, the progress of thereaction through these steps is of significant importance. Inparticular, for certain “real-time” nucleotide incorporation monitoringprocesses, the detectability of the incorporation event is improvedbased upon the amount of time the nucleotide is incorporated into andretained within the synthesis complex during its ultimate incorporationinto a primer extension product.

By way of example, in certain exemplary processes, the presence of thenucleotide in the synthesis complex is detected either by virtue of afocused observation of the synthesis complex, or through the use ofinteractive labeling techniques that produce characteristic signals whenthe nucleotide is within the synthesis complex. See, e.g., Levene, etal., Science 299:682-686, January 2003, and Eid, J. et al., Science,323(5910), 133-138 (2009), the full disclosures of which areincorporated herein by reference in their entirety for all purposes.

In the first exemplary technique, as schematically illustrated in FIG. 8, a nucleic acid synthesis complex, including a polymerase enzyme 802, atemplate sequence 804 and a complementary primer sequence 806, isprovided immobilized within an observation region 800, that permitsillumination (as shown by hv) and observation of a small volume thatincludes the complex without excessive illumination of the surroundingvolume (as illustrated by dashed line 808). By illuminating andobserving only the volume immediately surrounding the complex, one canreadily identify fluorescently labeled nucleotides that becomeincorporated during that synthesis, as such nucleotides are retainedwithin that observation volume by the polymerase for longer periods thanthose nucleotides that are simply randomly diffusing into and out ofthat volume.

In particular, as shown in panel II of FIG. 8 , when a nucleotide, e.g.,A, is incorporated into by the polymerase, it is retained within theobservation volume for a prolonged period of time, and upon continuedillumination yields a prolonged fluorescent signal (shown by peak 810).By comparison, randomly diffusing and not incorporated nucleotidesremain within the observation volume for much shorter periods of time,and thus produce only transient signals (such as peak 812), many ofwhich go undetected, due to their extremely short duration.

In particularly preferred exemplary systems, the confined illuminationvolume is provided through the use of arrays of optically confinedapertures termed zero-mode waveguides, e.g., as shown by confinedreaction region 100 (ZMWs)(See, e.g., U.S. Pat. No. 6,917,726, which isincorporated herein by reference in its entirety for all purposes). Forsequencing applications, the DNA polymerase is provided immobilized uponthe bottom of the ZMW (See, e.g., Korlach et al., PNAS U.S.A. 105(4):1176-1181. (2008), which is incorporated herein by reference in itsentirety for all purposes.

In operation, the fluorescently labeled nucleotides (shown as A, C, Gand T) bear one or more fluorescent dye groups on a terminal phosphatemoiety that is cleaved from the nucleotide upon incorporation. As aresult, synthesized nucleic acids do not bear the build-up offluorescent labels, as the labeled polyphosphate groups diffuses awayfrom the complex following incorporation of the associated nucleotide,nor do such labels interfere with the incorporation event. See, e.g.,Korlach et al., Nucleosides, Nucleotides and Nucleic Acids,27:1072:1083, 2008.

In the second exemplary technique, the nucleotides to be incorporatedare each provided with interactive labeling components that areinteractive with other labeling components provided coupled to, orsufficiently near the polymerase (which labels are interchangeablyreferred to herein as “complex borne”). Upon incorporation, thenucleotide borne labeling component is brought into sufficient proximityto the complex-borne (or complex proximal) labeling component, such thatthese components produce a characteristic signal event. For example, thepolymerase may be provided with a fluorophore that provides fluorescentresonant energy transfer (FRET) to appropriate acceptor fluorophores.These acceptor fluorophores are provided upon the nucleotide to beincorporated, where each type of nucleotide bears a different acceptorfluorophore, e.g., that provides a different fluorescent signal. Uponincorporation, the donor and acceptor are brought close enough togetherto generate energy transfer signal. By providing different acceptorlabels on the different types of nucleotides, one obtains acharacteristic FRET-based fluorescent signal for the incorporation ofeach type of nucleotide, as the incorporation is occurring.

In a related aspect, a nucleotide analog may include two interactingfluorophores that operate as a donor/quencher pair or FRET pair, whereone member is present on the nucleobase or other retained portion of thenucleotide, while the other member is present on a phosphate group orother portion of the nucleotide that is released upon incorporation,e.g., a terminal phosphate group. Prior to incorporation, the donor andquencher are sufficiently proximal on the same analog as to providecharacteristic signal, e.g., quenched or otherwise indicative of energytransfer. Upon incorporation and cleavage of the terminal phosphategroups, e.g., bearing a donor fluorophore, the quenching or other energytransfer is removed and the resulting characteristic fluorescent signalof the donor is observable.

Single-Molecule Sequencing Processes and Systems

In preferred aspects, the synthesis complexes in such reaction mixturesare arrayed so as to permit observation of the individual complexes thatare being so modulated. In arraying individual complexes to beindividually optically resolvable, the systems of the invention willposition the complexes on solid supports such that there is sufficientdistance between adjacent individual complexes as to allow opticalsignals from such adjacent complexes to be optically distinguishablefrom each other.

Typically, such complexes will be provided with at least 50 nm and morepreferably at least 100 nm of distance between adjacent complexes, inorder to permit optical signals, and particularly fluorescent signals,to be individually resolvable. Examples of arrays of individuallyresolvable molecules are described in, e.g., U.S. Pat. No. 6,787,308.

In some cases, individual complexes may be provided within separatediscrete regions of a support. For example, in some cases, individualcomplexes may be provided within individual optical confinementstructures, such as zero-mode waveguide cores. Examples of suchwaveguides and processes for immobilizing individual complexes thereinare described in, e.g., Published International Patent Application No.WO 2007/123763, the full disclosure of which is incorporated herein byreference in its entirety for all purposes.

As noted previously, in preferred aspects, the synthesis complexes areprovided immobilized upon solid supports, and preferably, uponsupporting substrates. The complexes may be coupled to the solidsupports through one or more of the different groups that make up thecomplex. For example, in the case of nucleic acid polymerizationcomplexes, attachment to the solid support may be through an attachmentwith one or more of the polymerase enzyme, the primer sequence and/orthe template sequence in the complex. Further, the attachment maycomprise a covalent attachment to the solid support or it may comprise anon-covalent association. For example, in particularly preferredaspects, affinity based associations between the support and the complexare envisioned. Such affinity associations include, for example,avidin/streptavidin/neutravidin associations with biotin or biotinylatedgroups, antibody/antigen associations, GST/glutathione interactions,nucleic acid hybridization interactions, and the like. In particularlypreferred aspects, the complex is attached to the solid support throughthe provision of an avidin group, e.g., streptavidin, on the support,which specifically interacts with a biotin group that is coupled to thepolymerase enzyme.

The sequencing processes, e.g., using the substrates described above andthe synthesis compositions of the invention, are generally exploited inthe context of a fluorescence microscope system that is capable ofilluminating the various complexes on the substrate, and obtaining,detecting, and separately recording fluorescent signals from thesecomplexes. Such systems typically employ one or more illuminationsources that provide excitation light of appropriate wavelength(s) forthe labels being used. An optical train directs the excitation light atthe reaction region(s) and collects emitted fluorescent signals anddirects them to an appropriate detector or detectors. Additionalcomponents of the optical train can provide for separation of spectrallydifferent signals, e.g., from different fluorescent labels, anddirection of these separated signals to different portions of a singledetector or to different detectors. Other components may provide forspatial filtering of optical signals, and focusing and direction of theexcitation and/or emission light to and from the substrate.

One such exemplary system is shown in FIG. 9 . An exemplary system isalso described in Lundquist et al., Published U.S. Patent ApplicationNo. 2007-0036511, Optics Letters, Vol. 33, Issue 9, pp. 1026-1028, thefull disclosure of which is incorporated herein by reference in itsentirety for all purposes.

Fluorescence reflective optical trains can be used in the applicationsof the systems of the invention. For a discussion on the advantages ofsuch systems, see, e.g., U.S. patent application Ser. No. 11/704,689,filed Feb. 9, 2007, Ser. No. 11/483,413, filed Jul. 7, 2006, and Ser.No. 11/704,733, filed Feb. 9, 2007, the full disclosures of which areincorporated herein by reference in their entirety for all purposes.

For purposes of the present invention, the processes and systems will bedescribed with reference to detection of incorporation events in a realtime, sequencing by incorporation process, e.g., as described in U.S.Pat. Nos. 7,056,661, 7,052,847, 7,033,764 and 7,056,676 (the fulldisclosures of which are incorporated herein by reference in theirentirety for all purposes), when carried out in arrays of discretereaction regions or locations. An exemplary sequencing system for use inconjunction with the invention is shown in FIG. 9 . As shown, the systemincludes a substrate 902 that includes a plurality of discrete sourcesof optical signals, e.g., reaction wells, apertures, or opticalconfinements or reaction locations 904. In typical systems, reactionlocations 904 are regularly spaced and thus substrate 902 can also beunderstood as an array 902 of reaction locations 904. The array 902 cancomprise a transparent substrate having a cladding layer on its topsurface with an array of nanoscale apertures extending through thecladding to the transparent substrate. This configuration allows for oneor more samples to be added to the top surface of the array, and for thearray to be observed through the transparent substrate from below, suchthat only the light from the apertures is observed. The array can beilluminated from below as shown in FIG. 9 , and in some embodiments, thearray can also be illuminated from above or from the side (not shown inFIG. 9 ).

For illumination from below, one or more excitation light sources, e.g.,lasers 910 and 920, are provided in the system and positioned to directexcitation radiation at the various signal sources. Here, two lasers areused in order to provide different excitation wavelengths, for examplewith one laser 910 providing illumination in the red, and another laser920 providing illumination in the green. The use of multiple laserexcitation sources allows for the optimal excitation of multiple labelsin a sample in contact with the array. The excitation illumination canbe flood illumination, or can be directed to discrete regions on thearray, for example, by breaking the excitation beam into an array ofbeamlets, each beamlet directed to a feature on the array. In order tobreak the excitation beams into an array of beamlets, a diffractiveoptical element (DOE) is used. In the system of FIG. 9 , the light fromexcitation sources 910 and 920 is sent through DOE components 912 and922 respectively. The use of a DOE for providing an array of beamlets isprovided, e.g. in U.S. Pat. No. 7,714,303, which is incorporated byreference herein in its entirety. Excitation light is then passedthrough illumination relay lenses 914 and 924 to interact with dichroic926. In the system of FIG. 9 , the red light from laser 910 is reflectedoff of dichroic 926, and the green light from laser 920 is directedthrough the dichroic 926. The excitation light is then passed throughillumination tube lens 928 into objective lens 970 and onto the array902.

Emitted signals from sources 904 are then collected by the opticalcomponents, e.g., objective 970, comprising dichroic element 975 whichallows the illumination light to pass through and reflects theexcitation light. The emitted light passes through collection tube lens930 and collection relay lens 932. The emitted light is then separatedinto 4 different spectral channels, and each spectral channel isdirected to a different detector. In the system of FIG. 9 , the light isseparated into four different channels, each channel correspondingpredominantly to one of four labels to be detected in the sample. Thus,the system allows the user to obtain four two-dimensional images, eachimage corresponding to one of the four labels. In order to separate thelight into the four spectral channels, dichroics 940, 942, and 944 areused. Dichroic 940 allows the light for channels 1 and 2 to pass whilereflecting the light for channels 3 and 4. Dichroic 942 allows the lightfor channel 1 to pass, through collection imaging lens 951 to detector961, and reflects the light for channel 2 through collection imaginglens 952 to detector 962. Dichroic 944 allows the light for channel 3 topass, through collection imaging lens 953 onto detector 963, andreflects the light for channel 4 through collection illumination lens954 onto detector 964. Each of the detectors 961-964 comprise arrays ofpixels. The detectors can be, for example, CMOS, EMCCD, or CCD arrays.Each of the detectors obtains 2-dimensional images of the channel thatis directed to that detector. The data from those signals is transmittedto an appropriate data processing unit, e.g., computer 970, where thedata is subjected to processing, interpretation, and analysis. The dataprocessing unit is configured to process the data both pixel by pixeland pixel region by pixel region, where each pixel region corresponds toa feature on the substrate. The data processing unit can receive datafrom calibration runs in order to define software mask pixel weighting,spectral weighting, and noise parameters. These parameters andweightings can be applied to signals that are measured on the detectorsduring an analytical reaction such as during sequencing. In someembodiments, the data processing unit is configured to define and applysoftware mask pixel weighting, spectral weighting, and noise parametersthat are determined and then applied during an analytical reaction suchas during sequencing.

Analyzed and processed data obtained from the analytical reactions canultimately be presented in a user ready format, e.g., on display 975,printout 985 from printer 980, or the like, or may be stored in anappropriate database, transmitted to another computer system, orrecorded onto tangible media for further analysis and/or later review.Connection of the detector to the computer may take on a variety ofdifferent forms. For example, in preferred aspects, the detector iscoupled to appropriate Analog to Digital (A/D) converter that is thencoupled to an appropriate connector in the computer. Such connectionsmay be standard USB connections, Firewire® connections, Ethernetconnections or other high speed data connections. In other cases, thedetector or camera may be formatted to provide output in a digitalformat and be readily connected to the computer without any intermediatecomponents.

This system, and other hardware descriptions herein, are provided solelyas a specific example of sample handling and image capture hardware toprovide a better understanding of the invention. It should beunderstood, however, that the present invention is directed to dataanalysis and interpretation of a wide variety of real-time fluorescentdetecting systems, including systems that use substantially differentillumination optics, systems that include different detector elements(e.g., EB-CMOS detectors, CCD's, etc.), and/or systems that localize atemplate sequence other than using the zero mode waveguides describedherein.

In the context of the nucleic acid sequencing methods described herein,it will be appreciated that the signal sources each represent sequencingreactions, and particularly, polymerase mediated, template dependentprimer extension reactions, where in preferred aspects, each baseincorporation event results in a prolonged illumination (orlocalization) of one of four differentially labeled nucleotides beingincorporated, so as to yield a recognizable pulse that carries adistinguishable spectral profile or color.

The present invention can include computer implemented processes, and/orsoftware incorporated onto a computer readable medium instructing suchprocesses, as set forth in greater detail below. As such, signal datagenerated by the reactions and optical systems described above, is inputor otherwise received into a computer or other data processor, andsubjected to one or more of the various process steps or components setforth below. Once these processes are carried out, the resulting outputof the computer implemented processes may be produced in a tangible orobservable format, e.g., printed in a user readable report, displayedupon a computer display, or it may be stored in one or more databasesfor later evaluation, processing, reporting or the like, or it may beretained by the computer or transmitted to a different computer for usein configuring subsequent reactions or data processes.

Computers for use in carrying out the processes of the invention canrange from personal computers such as PC or Macintosh® type computersrunning Intel Pentium processors, to workstations, laboratory equipment,or high speed servers, running UNIX, LINUX, Windows®, or other systems.Logic processing of the invention may be performed entirely by generalpurpose logic processors (such as CPU's) executing software and/orfirmware logic instructions; or entirely by special purposes logicprocessing circuits (such as ASICs) incorporated into laboratory ordiagnostic systems or camera systems which may also include software orfirmware elements; or by a combination of general purpose and specialpurpose logic circuits. Data formats for the signal data may compriseany convenient format, including digital image based data formats, suchas JPEG, GIF, BMP, TIFF, or other convenient formats, while video basedformats, such as avi, mpeg, mov, rmv, or other video formats may beemployed. The software processes of the invention may generally beprogrammed in a variety of programming languages including, e.g.,Matlab, C, C++, C#, Visual Basic, Python, JAVA, CGI, and the like.

While described in terms of a particular sequencing by incorporationprocess or system, it will be appreciated that certain aspects of theprocesses of the invention may be applied to a broader range ofanalytical reactions or other operations and varying systemconfigurations than those described for exemplary purposes.

In certain embodiments, the sequencing compositions described hereinwill be provided in whole, or in part, in kit form enabling one to carryout the processes described herein. Such kits will typically compriseone or more components of the reaction complex, such as the polymeraseenzyme and primer sequences. Such kits will also typically includebuffers and reagents that provide the catalytic and non-catalytic metalco-factors employed in the processes described herein. The kits willalso optionally include other components for carrying out sequencingapplications in accordance with those methods described herein. Inparticular, such kits may include ZMW array substrates for use inobserving individual reaction complexes as described herein.

In addition to the various components set forth above, the kits willtypically include instructions for combining the various components inthe amounts and/or ratios set forth herein, to carry out the desiredprocesses, as also described or referenced herein, e.g., for performingsequence by incorporation reactions.

Loading One Polymerase-Nucleic Acid Complex Per Zero Mode Waveguide

In some applications including in single-molecule sequencing, it isdesirable to deposit only a single polymerase-nucleic acid complex in azero mode waveguide. Conventionally, obtaining a single complex within azero mode waveguide in an array of zero mode waveguides is carried outstatistically by contacting the array of zero mode waveguides with adilute solution of polymerase-nucleic acid complexes at a concentrationsuch that a fraction of the zero mode waveguides are singly occupied, afraction are multiply occupied, and a fraction are unoccupied. Usingthis method there is a limit to the fraction of zero mode waveguidesthat will be singly occupied. The loading statistics can be modeled as aPoisson distribution, which predicts that a maximum of about 37% of thezero mode waveguides will be singly occupied.

The present invention provides for obtaining loading levels greater thanthat predicted by a Poisson distribution by producing a population ofpolymerase-nucleic acid complexes that are of a size such that once onecomplex becomes bound within a zero mode waveguide, the bound complexblocks the entry and binding of subsequent polymerase-nucleic acidcomplexes. This can be accomplished by first producing a population ofpolymerase-nucleic acid complexes and performing a controlled nucleicacid synthesis reaction which grows the nascent strand to a given lengthor range of lengths. The control of the length can be accomplished, forexample, but controlling the concentration of the reaction componentsand the time of the polymerase reaction. The control of the length ofthe nascent strand can also be accomplished by having a specific stoppoint in the template such that the nucleic acid synthesis is haltedwhen it reaches a given size.

Once the population of complexes with the desired size is produced, thecomplexes are attached to the zero mode waveguides. The attachment ofthe complexes can be accomplished as described herein. The polymerasewithin the complex can have a member of a binding pair attached to it(e.g. biotin or streptavidin), such that when exposed to the surfacehaving the other member of the binding pair, an attachment to thesurface is made. The attachment can be done either by exposing thesubstrate to a solution of the polymerase-nucleic acid complex andallowing the complexes to diffuse to the surface, or with a more activeloading process such as the bead loading described herein.

The use of circular templates and a strand displacing polymerase allowsfor the formation of a nascent strand that is longer than that of thetemplate. This can be useful for producing extended complexes that arelarge enough to block the entry of another complex into the zero modewaveguide. The zero mode waveguides can be, for example, cylinders withdiameters from about 50 nm to about 200 nm. The complex need notcompletely fill the zero mode waveguide to block the entry of anothercomplex. The blocking will be affected by the secondary and tertiarystructure of the extended complex. The tertiary structure can beaffected, for example, by the ionic composition and by the solventcharacteristics such as polarity and hydrogen bonding. These propertiescan be used to control how well a complex of a given length will blockthe zero mode waveguide from entry and binding of a second complex.

Exemplary Process for Attaching Complexes to Magnetic Beads and Loadingonto a zmw Chip

A library is produced having a plurality of double stranded fragments,the various fragments having sequences from portions of an original DNAsample. The plurality of double stranded fragments can be produced, forexample, by shearing or using restriction enzymes. The size distributioncan be controlled, for example, to give relatively long fragments—e.g.10 Kb or greater, or relatively small fragments—e.g. 200-300 bases.Hairpin adaptors are ligated onto the ends of the double strandedfragments to produce circular template molecules having a centraldouble-stranded portion and single-stranded hairpin loops at the ends(see SMRTbells™ from Pacific Biosciences®). The hairpin adaptors areprimed with primers having a 3′-poly(A) region. The primers hybridizewith the hairpin adaptor portion such that the complementary region ofthe primer hybridizes to the hairpin adaptor while the poly(A) portionremains unhybridized and single stranded. The solution of primedSMRTbell™ templates is exposed to phi-29 polymerase under conditions inwhich the polymerase-nucleic acid complex forms. This step is generallycarried out with an excess of polymerase e.g. 10:1 ratio of polymeraseto template at 1 nM, or a 3:1 ratio of polymerase to template at 10 nM.

A solution of magnetic beads having attached poly(T) DNA (e.g. Dynalbeads) is added to a tube. The beads are brought to the side of the tubewith a magnet and rinsed with buffer, e.g. once with high salt, and oncewith a buffer similar to that used for sequencing. Thepolymerase-nucleic acid complex is then added to the beads at theappropriate level of dilution (e.g. 20 pM), and the beads arere-suspended into this solution. The beads are in contact with thesolution to allow the poly(A) tails of the primers to hybridize to thepoly(T) groups on the beads. The level of attachment of the complexes tothe beads can be determined by fluorimetric methods.

The magnetic beads with polymerase-nucleic acid complex attached arethen washed one to three times with buffer or salt solution. The washsteps remove unattached complex, unwanted components, and uncomplexedenzyme. In the last step, the magnetic beads with complex are dispersedinto a sequencing reaction mixture. This solution can be stored for use,for example at 4° C., or can be dispensed directly onto a substrate. Thesolution can be dispensed onto a zmw chip having one or more permanentmagnets below the chip, and the magnets moved with respect to the chipto move the beads across the surface. In some cases, no magnet isrequired and gravity is used to load the complexes onto the chip. Theexposure to the chip can be, for example from 15 minutes to about 6hours. The shorter times can provide higher throughput, while the longertimes allow for the loading of lower concentrations of template, whichcan be useful where a minimal amount of sample is available.

Targeted Sequencing and Short Tandem Repeats

Although approximately 99.9% of human DNA sequences are the same inevery person, enough of the DNA is different to distinguish oneindividual from another, unless they are monozygotic twins. DNAprofiling can be done using repetitive (“repeat”) sequences that arehighly variable} referred to as variable number tandem repeats (VNTR).VNTRs loci are very similar between closely related humans, but sovariable that unrelated individuals are extremely unlikely to have thesame VNTRs.

A common method of DNA profiling with short tandem repeats (STR). usesPCR. The method uses highly polymorphic regions that have short repeatedsequences of DNA (the most common is 4 bases repeated, but there areother lengths in use, including 3 and 5 bases). Because unrelated peoplealmost certainly have different numbers of repeat units, STRs can beused to discriminate between unrelated individuals. These STR loci(locations on a chromosome) are targeted with sequence-specific primersand amplified using PCR. The DNA fragments that result are thenseparated and detected using electrophoresis. Each STR is polymorphic,however, the number of alleles is very small. Typically each STR allelewill be shared by around 5-20% of individuals. The power of STR analysiscomes from looking at multiple STR loci simultaneously. The pattern ofalleles can identify an individual quite accurately. Thus STR analysisprovides an excellent identification tool. The more STR regions that aretested in an individual the more discriminating the test becomes.

One aspect of the invention is a method of DNA profiling using hookoligonucleotides and the isolation of active enzyme complexes havingspecific portions of DNA. This is a resequencing application, so oneknows a priori the relevant sequences for designing oligos to target thesequences. The regions of interest are well defined and relatively smallin number. These method described below takes advantage of theversatility and granularity of single molecule real time sequencing. Themethods described herein also can have a short time to result, which canbe critical for this DNA profiling applications. Further, our lack of GCbias in SMRT real time sequencing facilitates reading through GC-richregions including STRs. Since DNA fragments are measured at the singlemolecule level, other data including relative copy number between STRsand variation within a single STR, or heterogonous mutations are alsoavailable. An exemplary method is shown in FIG. 22 . For sample prep, 1.Double stranded genomic DNA is sheared to a size amenable to ZMW loadingby beads such as magnetic beads. The size can be, for example, on theorder of 10 Kb. 2. Capture oligos, comprising initiation, linker andhook domains (of total length 20-100 bases), covering all desired STRregions, are added, the sample is heated and then slow cooled tofacilitate binding. The oligos are designed such that the only 3′ endavailable is at initiation site. 3. Polymerase is added and the “bindingtube” is formed. 4. A short walk-in sufficient to knock off the hookregion is completed. The short walk in can be accomplished bycontrolling the kinetics and time of polymerase activity (Sr, Ca, orshort duration of Mg or Mn), or by using stop sites or regions. 5. Beadswith the complementary sequence are added and used to pull-down properlyformed and active complex 6. Sample is loaded into ZMWs and sequenced asdescribed herein.

In some cases for improved coverage and accuracy, pairs of oligos perSTR can be added, one for each strand, going in opposite directions.This would provide redundant information improving data quality.

Single-Strand SMRTbell Capture Using Modified Bases

One method of enriching a DNA sample for a specific DNA modification ofinterest (5-mC. 5-hmC, 8-oxoG, etc.) is to use antibodies or specificbinding proteins that are attached to beads to pull down regions of DNAcontaining the modification. Following capture, the region surroundingthe DNA modification can be sequenced to identify the genomic region.SMRT DNA sequencing, in comparison to second generation sequencingmethods, allows for direct detection of DNA modifications at singlenucleotide resolution (Flusberg et al. Nature Methods 2010). Directdetection requires sequencing of the native DNA (non-amplified).Standard SMRTbell library preparation however, relies on double-strandedDNA. Furthermore, SMRTbells are less amenable to standard denaturingmethods because they have a high propensity to re-anneal. However, itmay be possible to make one strand of a SMRTbell stably single strandedusing a DNA polymerase.

One aspect of the invention is a method of obtaining a single strandedtemplate for capture by the following method. This single strandedregion is then isolated using a pull-down method targeted to themodified bases. First, a DNA sample of interest that contains a specificmodification (5-methyl C, for example) is obtained. The DNA isfragmented and converted into a SMRTbell library (circular DNA havingcentral double stranded regions and single stranded hairpins on eachend. Some portion of the SMRTbell molecules will contain themodification of interest. Capture methods using antibodies against5-methyl C (for example) require that the DNA is single stranded.

To make a SMRTbell into a stably single stranded form, first a DNApolymerase (e.g. phi29) is bound to a SMRTbell that has been primerannealed onto its single stranded region using standard methods. Thepolymerase is given appropriate conditions to allow slow walking (e.g.calcium buffer) as described herein. The amount of time can becontrolled and will depend on the size of the fragments in the SMRTbelllibrary. Once the polymerase has made it about one half lap around theSMRTbell template (gone from one hairpin to the other hairpin), thepolymerase is stopped using appropriate buffer conditions (i.e.strontium buffer) or an appropriate modification to the template thatcauses the polymerase to stop at the opposite hairpin. FIG. 23 shows howthis process results in one strand of the SMRTbell being singlestranded. This strand is now available for binding and pull-down. Afterpull down, the captured population of SMRTbells can be sequenced usingstandard SMRT™ sequencing methods. This type of sequencing can detectthe sites of DNA modification using polymerase kinetics.

The walk-in can be controlled as described herein either by controllingthe rate and time of the polymerase reaction, or by including sequenceregions or modified bases in the hairpin region in order to cause thepolymerase to stop. It will be understood that the modified bases thatare exposed and pulled down in this method are generally different thanthe modified bases that are used to halt the polymerase within thehairpin region.

In some cases, the pull-down of the template with the modified base iscarried out such that the polymerase remains active. When done in thismanner, the isolated complex can be loaded into zero mode waveguides forsequencing. In some cases, the isolated templates comprising modifiedbases are separated from the polymerase enzyme used for walk-in. Inaddition the isolated template can be treated to remove the extendedprimer and re-generate the SMRTbell™. This can be done by annealing thesample to dissociate the extended primer. The removal of the extendedprimer can also be accomplished by treatment with exonuclease which willselectively degrade the single stranded portion, leaving the circularSMRTbell™ intact.

The modified bases can be any suitable modified nucleobase. The modifiedbases can be naturally occurring modified bases, which are present inmany if not all organisms and are used, for example to regulate geneexpression. The modified bases can also be non-natural modified basessuch as those that occur due to synthetic chemical reactions or due toexposure to environmental factors such as ultraviolet light or oxygen.The modified bases include, for example, 5-methylcytosine,N⁶-methyladenosine, etc.), pseudouridine bases, 7,8-dihydro-8-oxoguaninebases, 2′-O-methyl derivative bases, base J, base P, base Z, s4U, s6G,nicks, apurinic sites, apyrimidic sites, non-canonical bases or basepairs, pyrimidine dimers, a cis-platen crosslinking products, oxidationdamage, hydrolysis damage, bulky base adducts, thymine dimers,photochemistry reaction products, interstrand crosslinking products,mismatched bases, secondary structures, and bound agents. Suitablemodified bases are described, for example, in U.S. patent applicationSer. No. 12/945,767, filed Nov. 10, 2010, the contents of which areincorporated by reference herein in its entirety for all purposes. Anybinding protein or antibody that is specific for one or more of thesemodified bases can be used to isolate the template molecule containingthese bases. Proteins that can be used to bind selectively to methylatedsites include RNA and DNA polymerases, reverse transcriptases, histones,nucleases, restriction enzymes, replication protein A (RPA),single-stranded binding protein (SSB), RNA-binding proteins,microRNA-containing ribonucleoprotein complexes, anti-DNA antibodies,DNA damage-binding agents, modifying agents, agents that bind alterednucleotides (e.g., methylated), small RNAs, microRNAs, drug targets,etc.

There are a number of proteins that can bind stably and specifically tomethylated DNA including members of the MBD family of human proteins,all of which contain a methyl-CpG binding domain (MBD). For example,MECP2, MBD1, MBD2, and MBD4 all bind specifically to methylated DNA, andare involved in repressing transcription from methylated gene promoters.Binding of these proteins to a template nucleic acid is expected tocause a translocating polymerase to pause proximal to the bound protein.As such, an increased pause duration during single-molecule sequencingreactions is indicative of a methylated base in the template nucleicacid. It is therefore important that the protein bind tightly to itstarget nucleic acid sequence. Natural MBD proteins only have micromolarKd affinities for methyl-CpG sequences, so engineered MBD proteins thatbind more tightly to the methylated template sequence can enhancedetectability of methylated bases. For example, a multimerized MBD1protein is provided in Jorgensen, et al., Nucleic Acids Research 2006,34(13), e96. Such engineered proteins can have a single methyl bindingdomain with a lower Kd (sub-micromolar) or multiple methyl-bindingdomains that increase the effective concentration of the methyl-bindingdomain in the vicinity of the methylated DNA template. More informationon the MBD family of proteins is provided, e.g., in B. Hendrich, et al.,Mol Cell Biol 1998, 18(11), 6538; and I. Ohki, et al., EMBO J 2000,18(23), 6653.

In addition, the mammalian UHRF1 (ubiquitin-like, containing PHD andRING finger domains 1) protein binds tightly to methylated DNA and isrequired for its maintenance. Crystal structures of the SRA domain ofthis protein bound to DNA show that the 5-MeC is flipped out of the DNAduplex and stabilized by hydrophobic stacking and hydrogen bonding toSRA protein residues. (See, e.g., G. V. Avvakumov, et al. and H.Hashimoto, et al., both supra.) Further, McrBC is an endonuclease thatcleaves DNA containing 5-methylcytosine or 5-hydroxymethylcytosine orN4-methylcytosine on one or both strands, but does not act uponunmethylated DNA. McrBC requires GTP for cleavage, but in the presenceof a non-hydrolyzable analog of GTP, the enzyme will bind to methylatedDNA specifically, without cleavage. (See, e.g., Irizarry, R. A. et al.(2008) Genome Res., 18, 780-790; and Hublarova, P. et al. (2009) Int JGynecol Cancer, 19, 321-325, the disclosures of which are incorporatedherein by reference in their entireties for all purposes.) Finally, themonoclonal antibody to 5-MeC, used for methylated DNAimmunoprecipitation, also binds specifically to methylated cytosine.(See, e.g., N. Rougier, et al., Genes Dev 1998, 12, 2108; and M. Weber,et al., supra, which are incorporated herein by reference in theirentireties for all purposes.)

In yet further embodiments antibodies can be used to target the specificmodified bases. For example, an antibody against 5-MeC could be used tobind 5-MeC in a template nucleic acid, similar to the process used inmethylated DNA immunoprecipitation assays (M. Weber, et al., Nat Genet2005, 37, 853).

Thus, in some aspects, the invention provides methods for isolating DNAhaving a modified or unnatural base comprising: obtaining a library ofcircular DNA fragments each comprising a double stranded DNA centralregion and single stranded regions on the ends of the double strandedregions wherein at least some of the fragments comprise a modified orunnatural base; treating the DNA fragments with a primer and apolymerase under conditions where the polymerase extends the primer tocopy at least one of the strand of the double stranded region so as torender the other strand of the double-stranded portion single stranded;using a binding protein or antibody that is specific to the modified orunnatural base to isolate strands containing the modified or unnaturalbases from those that do not contain the unnatural or modified bases. Insome embodiments, the modified or unnatural base is methyl-C,hydroxy-methyl C, or oxo-G.

EXAMPLES Example 1: Hook Capture Using a Common Hook

Polymerase-SB Template Complex Formation:

Several types of SMRTBell™ (SB) templates having the common (VD) Adaptor5′-TCTCTCTCTTTTCCTCCTCCTCCGTTGTTGTTGTTGAGAGAGAT-3′ (SEQ ID NO: 1) (FIG.10(A)), 2k (Phix3-12), 1k (PhiX9-16), 0.6 k (Bsub1), and 2k λ-library,were individually mixed with SA-Pol (3029P) for the Pol-template bindingtubes. The condition for the binding was: 3 nM SB template, 15 nMSA-Pol, 50 mM Tris acetate, pH 8.0, 0.05% (v/v) Tween-20, 20 mMpotassium acetate, 0.2 mM calcium acetate, 1 μM each dNTP, and 100 mMDTT. Volume in each tube was either 0.2 mL or 1 mL. These tubes wereincubated at 22° C. for approximately 16 hours. To each tube, 0.05volume of 20 mM Strontium acetate (10 μL Sr for 200 μL tube) was added.

Samples for Enrichment:

-   -   Sample 1: A mixture of 143 pM Pol-2k SB (PhiX3-12) and 2857 pM        Pol-0.6k SB (Bsub1).    -   Sample 2: A mixture of 136 pM Pol-2k SB (PhiX3-12), 136 pM        Pol-1k SB (PhiX9-16) and 2723 pM Pol-0.6k SB (Bsub1).    -   Sample 3: A mixture of 150 pM Pol-2k SB (PhiX3-12) and 3000 pM        Pol-2k λ-library.    -   Sample 4: A mixture of 110 pM Pol-2k SB (PhiX3-12), 2800 pM        Pol-0.6k SB (Bsub1) and 2300 pM Pol-2k λ-library.        Bead-Based Purification of Active Complexes:

Samples 1 and 2 were diluted in an equal volume of BBB (50 mM Trisacetate, pH 8.0, 0.05% (v/v) Tween-20, 100 mM potassium acetate, 1 mMstrontium acetate, 0.5 μM each dNTP, and 10 mM DTT). Heparin (Sigma, cat#H4784) was added to final concentration of 1 mg/mL. The tubes wereincubated for 10 min at 22 C and 30 min on ice. Potassium acetate wasadded to the final concentration of 0.1 M. The “VD-hook” oligo was addedat 20-fold higher of hook concentration than the SB templateconcentration. (VD-hook: 5′-TCTCTCTCAACAA(A)23-3′ (SEQ ID NO: 2). Thetubes were incubated at 4° C. for 2 to 16 hr. FIG. 10 (B) shows a tablewith calculated melting temperatures for hybridized oligonucleotides atvarious salt concentrations. It can be seen, for example, that themelting temperature of the specific 13-mer portion of the VD-hook can bevaried over a wide range of melting temperatures by changing the saltconcentration. The table also shows some calculated melting temperaturesfor 20mer and 15mer poly A oligonucleotides. By changing the length ofthe poly(A) on the retrieval part of the oligonucleotide, the relativestrength of these links can be controlled. Generally, the smallerdifference between the melting temperature and the sample temperature,the weaker the bond. As described above, the hook oligo can be designedsuch that the bond between the capture portion of the hookoligonucleotide and the template nucleic acid is stronger than the bondbetween the retrieval portion of the hook oligonucleotide and the bead.

Oligo (dT)25 magnetic beads from New England Biolabs (cat #S1419S) waswashed and equilibrated in BBB. For each volume (μL) of samplecontaining 200 fmoles of hook oligo, 1 μL of beads was used. The sampleswith beads were mixed well and kept on ice for 1 hour. The beads werepulled to the side of tube using a magnetic stand (Invitrogen), theliquid was discarded. Then 0.2 mL of cold BBB was added to each tube,and the beads were mixed well. Again, the beads were pulled to the sideof tube using a magnetic stand, and the liquid was discarded. The beadswere washed with BWB2 (50 mM Tris acetate, pH 8.0, 0.05% (v/v) Tween-20,400 mM potassium acetate, 1 mM strontium acetate, 0.5 μM each dNTP, and10 mM DTT), and then with BBB. The purified active Pol-templatecomplexes were eluted in BEB (50 mM Tris acetate, pH 8.0, 0.05% (v/v)Tween-20, 10 mM potassium acetate, 1 mM strontium acetate, 0.5 μM eachanalog (dye-dN6P), and 10 mM DTT). The tubes were incubated at 30° C.for 10 min; they were immediately placed on a magnetic stand and theliquid fractions containing the purified active Pol-template complexeswere transferred to new tubes.

The DNA concentrations of the purified complexes were determined by afluorescent assay using a kit (Quant-iT™ DNA Assay Kit, High Sensitivityfrom Invitrogen).

4-Color Single Molecule Sequencing:

The purified complexes were used for single molecule sequencingaccording to the method described in Eid et al., Science Vol. 323 no.591.0 pp. 133-138 (January 2009).

Example 2: Hook Capture for Enrichment of Specific Sequences

Polymerase-SB Template Complex Formation:

Several types of SB templates, 2k (Phix3-12), 1k (PhiX9-16), 0.6 k(Bsub1), and 2k λ-library, were individually mixed with SA-Pol (3029P)for the Pol-template binding tubes. The condition for the binding was: 3nM SB template, 15 nM SA-Pol, 50 mM Tris acetate, pH 8.0, 0.05% (v/v)Tween-20, 20 mM potassium acetate, 0.2 mM calcium acetate, 1 μM eachdNTP, and 100 mM DTT. Volume in each tube was either 0.2 mL or 1 mL.These tubes were incubated at 22° C. for approximately 16 hours. To eachtube, 0.05 by volume of 20 mM Strontium acetate (10 μL Sr for 200 μLtube) was added.

Samples for Enrichment:

-   -   Sample 1: A mixture of 143 pM Pol-2k SB (PhiX3-12) and 2857 pM        Pol-0.6k SB (Bsub1).    -   Sample 2: A mixture of 136 pM Pol-2k SB (PhiX3-12), 136 pM        Pol-1k SB (PhiX9-16) and 2723 pM Pol-0.6k SB (Bsub1).    -   Sample 3: A mixture of 150 pM Pol-2k SB (PhiX3-12) and 3000 pM        Pol-2k λ-library.    -   Sample 4: A mixture of 110 pM Pol-2k SB (PhiX3-12), 2800 pM        Pol-0.6k SB (Bsub1) and 2300 pM Pol-2k λ-library.        Bead-Based Purification of Active Complexes:

Samples 1 and 2 were diluted an equal volume of BBB (50 mM Tris acetate,pH 8.0, 0.05% (v/v) Tween-20, 100 mM potassium acetate, 1 mM strontiumacetate, 0.5 μM each dNTP, and 10 mM DTT). Heparin (Sigma, cat #H4784)was added to final concentration of 1 mg/mL. The tubes were incubatedfor 10 min at 22 C and 30 min on ice. Potassium acetate was added to thefinal concentration of 0.1 M. The specific “hook” oligo was added at20-fold higher of hook concentration than the SB template concentration.(2k-L-hook: 5′-AATGCTTACTCAAG(A)23-3′(SEQ ID NO: 3; 2k-R-hook:5′-ATGAAGTAATCACG(A)23-3′(SEQ ID NO: 4)). The tubes were incubated at 4°C. for 2 to 16 hr.

Oligo (dT)25 magnetic beads from New England Biolabs (cat #S1419S) werewashed and equilibrated in BBB. For each volume (μl) of samplecontaining 200 fmoles of hook oligo, 1 μL of beads was used. The sampleswith beads were mixed well and kept on ice for 1 hour. The beads werepulled to the side of tube using a magnetic stand (Invitrogen), and theliquid was discarded. Then 0.2 mL of cold BBB was added to each tube,the beads were mixed well. Again, the beads were pulled to the side oftube using a magnetic stand, liquid was discarded. The beads were washedwith BWB2 (50 mM Tris acetate, pH 8.0, 0.05% (v/v) Tween-20, 400 mMpotassium acetate, 1 mM strontium acetate, 0.5 μM each dNTP, and 10 mMDTT), and then with BBB. The purified active Pol-template complexes wereeluted in BEB (50 mM Tris acetate, pH 8.0, 0.05% (v/v) Tween-20, 10 mMpotassium acetate, 1 mM strontium acetate, 0.5 μM each analog(dye-dN6P), and 10 mM DTT). The tubes were incubated at 30° C. for 10min; then they were immediately placed on a magnetic stand and theliquid fractions containing the purified active Pol-template complexeswere transferred to new tubes.

The DNA concentrations of the purified complexes were determined by afluorescent assay using a kit (Quant-iT™ DNA Assay Kit, High Sensitivityfrom Invitrogen).

4-Color Single Molecule Sequencing:

The purified complexes were used for single molecule sequencingaccording to the method described in Eid et al., Science Vol. 323 no.5910 pp. 133-138 (January 2009).

Results:

The yields of specifically sequenced templates from Sample 1 (2templates) and Sample 2 (3 templates) were compared. FIG. 11 graphicallydisplays some of the results, showing the sequencing yield (number ofreads or nReads) for Sample 1 with no hook purification, Sample 2 with acommon hook purification, Sample 1 purified using a hook moleculetargeted to one region of 2k SB (PhiX3-12) (2k-L-hook), and Sample 2purified using a hook molecule targeted to another region of 2k SB(PhiX3-12) (2k-R-hook). The enrichment of the 2k SB (PhiX3-12) usingspecific hooks targeting this template is compared to using a commonhook molecule targeting all 3 templates (the VD-hook). Sample 1 has20-fold excess of 0.6k SB (Bsub1); the purified sample 1 using specifichook for 2k SB (PhiX3-12) showed 300-fold more reads of 2k SB than thenumber of reads for 0.6 k SB. Therefore, the enrichment of 2k SB forsample 1 is about 6000-fold.

FIG. 12 compares the relative yield (fraction Reads) of specificallysequenced templates from Sample 3 (2 template types) and Sample 4 (3template types). The enrichment of the 2k SB (PhiX3-12) using a specifichook targeting this template is compared to using the common VD-hooktargeting all 3 templates. Sample 3 has 20-fold excess of 2k λ-libraryof SB; the purified sample 3 using specific hook for 2k SB (PhiX3-12)showed 20-fold more reads of 2k SB (phiX3-12) than the number of readsfor 2k λ-library SB. Therefore, the enrichment of 2k SB for sample 3 isabout 400-fold. Sample 4 has ˜49-fold less of 2k SB (PhiX3-12) than thenumbers of 2k λ-library plus 0.6k SB (Bsub1); the bead-purified sample 4using specific hook for 2k SB (PhiX3-12) showed ˜19-fold more reads of2k SB (phiX3-12) than the number of reads for 2k λ-library and Bsub1 SB.Therefore, the enrichment of 2k SB for sample 4 is about 900-fold.

Example 3: Deposition of Polymerase-Nucleic Acid Complexes with MagneticBeads

A SMRTBell™ (SB) template, 2k (Phix3-12), was mixed with SA-Pol (3029P)for the Pol-template binding tubes. The condition for complex bindingwas: 3 nM SB template, 9 nM SA-Pol, 50 mM Tris acetate, pH 8.0, 0.05%(v/v) Tween-20, 20 mM potassium acetate, 0.2 mM calcium acetate, 1 μMeach dNTP, and 100 mM DTT. The volume of the tube was 0.2 mL. The tubewas incubated at 30° C. for 4 hours and subsequently kept at 4° C. untilready for testing (typically within 12 hours). The contents of the tubewere then split into two separate tubes of equal volume, 0.1 mL each.One tube (Sample 1) served as the control and no further modificationswere made. The other tube (Sample 2) was hooked to magnetic beads in thefollowing manner.

Sample Hook:

Heparin (Sigma, cat #H4784) was added to Sample 2 to a finalconcentration of 1 mg/mL and Sr was added to a final concentration of 1mM. The tube was incubated for 30 min on ice. Heparin was employed totrap free polymerase and Sr served to stop the “walk-in” of thepolymerase on the DNA template. Potassium acetate was then added to afinal concentration of 0.1 M and the “hook” oligo was added at aconcentration of 60 nM. The sample tube was incubated at 4° C. for 16hrs. This sample (Hooked Sample 2) was now ready for bead attachment.

Bead Preparation and Attachment:

In a separate tube, 80 uL of oligo (dT)25 magnetic beads from NewEngland Biolabs (cat #S1419S) were washed and equilibrated in BBB. Thebeads were pulled to the side of tube using a magnetic stand(Invitrogen) and the liquid was discarded. Then, 0.1 mL of Hooked Sample2 was added to the bead tube and mixed well. The Hooked Sample 2 andbeads were stored @ 4 C for 1 hr to allow for attachment. Then newlycomplexed beads were purified in three subsequent steps. Purificationstep 1: The beads were pulled to the side of tube using a magneticstand, and the liquid was discarded. Purification step 2: The beads werewashed with BWB2 (50 mM Tris acetate, pH 8.0, 0.05% (v/v) Tween-20, 400mM potassium acetate, 1 mM strontium acetate, 0.5 μM each dNTP, and 10mM DTT). Purification step 3: Repeat of step 1. After purification, thebeads were pulled to the side of tube using a magnetic stand, the liquidwas discarded, and the complexed beads were resuspended in BBB at 0.07mL final volume, after which and this sample (Complexed Bead Sample) wasready for testing.

Prior to loading onto the surface of a ZMW array chip, both the Sample 1Control (S1C) and the Complexed Bead Sample (CBS) were diluted todesired concentrations in BBB. ZMW array chips were prepared for sampleloading by priming 2× with 25 uL of 50 mM MOPS pH7.5. For each sample, a25 uL aliquot of diluted sample was pipetted onto the primed chipsurfaces and the aliquot was incident for a total loading time of 5minutes. The S1C sample loaded by diffusion and no further modificationswere necessary. For the CBS, magnetic loading was employed, where apermanent magnet was passed under the sample at an approximate rate of 1pass/10 s. In this manner, the beads can be seen traversing the arraysurface with each pass. After 5 minutes, the aliquots were removed andthe chip was washed 5× in a wash buffer (50 mM Tris acetate, pH 8.0, 100mM KOAc, 40 mM DTT). Following washing, reagents for DNA sequencing wereplaced on the chip and a sequencing run was performed to assess theloading of the ZMW arrays. Loading activity was quantified using thepercentage of ZMWs that showed statistically significant sequencingactivity (Z>3) via alignments to the reference DNA template whensequenced by the method described by Eid, J. et al., Science, 323(5910),133-138 (2009).

FIG. 13 shows a comparison of loading between the magnetic bead anddiffusion loading. Magnetic bead loading results in higher loading for agiven amount of input DNA. At an identical DNA concentration (150 pM),the bead loading is 20× more efficient than diffusion.

Example 4: Higher Representation of Large Templates when Loading ofPolymerase-Nucleic Acid Complexes with Magnetic Beads

Experimental Procedure:

A synthetic library was made from pCYPAC2 (18.7 kbp), where the organismDNA was cut into strands of varying length and subsequently made intoSMRTbells as described above. The SB ranged in size from 160 bp to 4251bp. The synthetic library was mixed with SA-Pol (3029P) for thePol-template binding tubes. The condition for complex binding was: 3 nMSB template, 9 nM SA-Pol, 50 mM Tris acetate, pH 8.0, 0.05% (v/v)Tween-20, 20 mM potassium acetate, 0.2 mM calcium acetate, 1 μM eachdNTP, and 100 mM DTT. The volume of the tube was 0.2 mL. The tube wasincubated at 30° C. for 4 hours and subsequently kept at 4° C. untilready for testing (typically within 12 hours). The contents of the tubewere then split into two separate tubes of equal volume, 0.1 mL each.One tube (Sample 1) served as the control and no further modificationswere made. The other tube (Sample 2) was hooked to magnetic beads in thefollowing manner.

Sample Hook:

Heparin (Sigma, cat #H4784) was added to Sample 2 to a finalconcentration of 1 mg/mL and Sr was added to a final concentration of 1mM. The tube was incubated for 30 min on ice. Heparin was employed totrap free polymerase and Sr served to stop the “walk-in” of thepolymerase on the DNA template. Potassium acetate was then added to thefinal concentration of 0.1 M and the “hook” oligo was added at aconcentration of 60 nM. The sample tube was incubated at 4° C. for 16hrs. This sample (Hooked Sample 2) was now ready for bead attachment.

Prior to loading onto the surface of a ZMW array chip, both the Sample 1Control (SIC) and the Complexed Bead Sample (CBS) were diluted todesired concentrations in BBB. ZMW array chips were prepared for sampleloading by priming 2× with 25 uL of 50 mM MOPS pH7.5. For each sample, a25 uL aliquot of diluted sample was pipetted onto the primed chipsurfaces and the aliquot was incident for a total loading time of 5minutes. The S1C sample loaded by diffusion and no further modificationswere necessary. For the CBS, magnetic loading was employed, where apermanent magnet was passed under the sample at an approximate rate of 1pass/10 s. In this manner, the beads can be seen traversing the arraysurface with each pass. After 5 minutes, the aliquots were removed andthe chip was washed 5× in a wash buffer (50 mM Tris acetate, pH 8.0, 100mM KoAc, 40 mM DTT). Following washing, reagents for DNA sequencing wereplaced on the chip and a sequencing run was performed to assess theloading of the ZMW arrays. Loading activity was quantified using thepercentage of ZMWs that showed statistically significant sequencingactivity (Z>3) via alignments to the reference DNA template.

FIG. 14 shows a comparison of loading between the magnetic bead anddiffusion loading. Diffusion loading has a distinct bias towards loadingsmaller templates. In contrast, magnetic bead loading shows relativelyeven loading for all tested template sizes.

Example 5: Re-Use of Polymerase-Nucleic Acid Complexes on Magnetic Beads

Complexed Bead Samples were prepared in the manner as described above.Magnetic loading was employed, where a permanent magnet was passed underthe sample at an approximate rate of 1 pass/10 s. In this manner, thebeads can be seen traversing the array surface with each pass. After 5minutes, the beads were removed and transferred onto a new freshlyprimed chip. The loaded chip was washed 5× in a wash buffer (50 mM Trisacetate, pH 8.0, 100 mM KoAc, 40 mM DTT). Following washing, reagentsfor DNA sequencing were placed on the chip and a sequencing run wasperformed to assess the loading of the ZMW arrays. This process wasrepeated 5× to assess the re-usability of the complexed bead samples.Loading activity was quantified using the percentage of ZMWs that showedstatistically significant sequencing activity (Z>3) via alignments tothe reference DNA template.

FIG. 15 shows that complexed bead samples can be re-used several timeswith a small loss in loading efficiency. Each re-use of complexed beadsamples shows a loading efficiency loss of approximately 10%.

Example 6: Producing and Retrieving a Linear Double Stranded TemplateHaving a Gap in One Strand

A population of DNA fragments having blunt ends is produced by shearinga DNA sample and polishing the fragments to form blunt ends. The bluntended fragments are then ligated to the linear adaptor below:

5′-CAACGGAGGAGGAGG-3′ 5′-GGACCACCTCCTGAGAGAGAGA-3′3′-NGTTGCCTCCTCCTCCNNNNNNNNNNTGGTGGAGGACTCTCTCTCT- 5′Made up of oligos:

(SEQ ID NO: 5) 5′-CAACGGAGGAGGAGG-3′ (SEQ ID NO: 6)5′-GGACCACCTCCTGAGAGAGAGA-3′ (SEQ ID NO: 7)5′-TCTCTCTCTCAGGAGGTGGTNNNNNNNNNNCCTCCTCCTCCGTTGN- 3′

For this sequence, “N” can be either A, C, G or T as this sequence isnot important for hybridization of the oligos involved. Generally, thepoly N region is designed such that it will not hybridize to the capturesequence of a hook oligonucleotide. It is also designed such that itwill not be complementary to any of the three oligonucleotides that makeup the linear adaptor.

To the resulting library of fragments is added a phi-29 DNA polymeraseenzyme in excess under conditions whereby binding of the polymeraseenzyme to the fragments occurs. Nucleic acid synthesis is initiated suchthat the polymerase enzyme produces a growing strand from the 3′position within the gap, displacing the strand ahead of it. Nucleic acidsynthesis is halted by the addition of a solution of Sr ions. A hookoligonucleotide having the structure below:

(SEQ ID NO: 8) 5′-AGGAGGTGGTCC(A)23-3′

having a 5′ end that is complementary to a portion of the displacedstrand in the active complexes is added under conditions providing forselective hybridization. Magnetic beads having bound poly(T)oligonucleotides are added to bind to the poly(A) region of the hookoligonucleotide. Permanent magnets are used to hold the magnetic beadsin place while wash solutions are used to remove components of themixture not bound to the beads. The isolated polymerase-nucleic acidcomplexes can then be deposited onto substrates for single moleculeanalysis, either by eluting the complexes from the beads using theappropriate levels of salt and temperature and exposing the elutedcomplexes to the substrates; or by contacting the magnetic beads havingpolymerase-nucleic acid complex bound thereto with the substrates usingmagnetic fields.

Example 7: Purifying Polymerase-Nucleic Acid Complex Using a SingleStranded Region

Annealing of a Poly-A Tailed Primer to SMRTbell™ Template

An oligonucleotide primer is annealed to both hairpins of a SMRTbelltemplate. The primer consists of a 5′ polyA-tract and 3′ regioncomplementary to the SMRTbell hairpin. The annealing reaction isassembled in a buffer solution consisting of 50 mM Tris-Acetate(“TOAc”), 20 mM Potassium Acetate (“KOAc”), and 0.05% Tween-20. Theprimer is provided at approximately twice the molar concentration of thetemplate.

Binding Polymerase to SMRTbell Template

Primed SMRTbell templates are combined with nucleotides, an inhibitorymetal cation (Calcium or Strontium), DTT, and the DNA polymerase to beused for single-molecule sequencing. The polymerase is typicallyprovided at several fold higher concentration than the template, toensure binding of active polymerase molecules at all priming sites. Thebinding reaction is incubated at 30° C. for 30-240 minutes.

Complex Purification Via Primer Capture

Polymerase-bound SMRTbell molecules (ternary “complexes”) are preparedfor bead purification by the addition of heparin (to bind freepolymerase) and adjustment of salt concentration to 100 mM KOAc (foroptimal nucleic acid hybridization while maintaining polymerase-SMRTbellbinding), and incubated for 15-30 minutes on ice. Commercially availablemagnetic beads (conjugated with a poly-T oligonucleotide), areintroduced to the complexes, and allowed to bind for 15-30 minutes onice. The magnetic beads (now bound to the primer via polyA::polyThybridization) are washed with several wash buffers, prior to a finalwash in 100 mM KOAc binding buffer. At this point, the beads may bestored in the 100 mM KOAc binding buffer for later magnetic loading ofZMWs, or active sequencing complexes may be eluted from the beads.Elution is performed by introducing a low salt elution buffer (˜5 mMKOAc) and a short incubation at 30° C. The beads are discarded, and thesupernatant fraction contains active sequencing complexes. Theconcentration of DNA can be measured in this eluate via a fluorimetricmethod such as PicoGreen dye binding.

Example 8: Stability of Isolated Polymerase-Nucleic Acid Complexes

A set of experiments was performed to determine the stability ofisolated polymerase-nucleic acid complexes. A 2 Kb library of circulartemplates from E. coli comprising double stranded regions flanked byhairpin adaptors at the ends was prepared. The templates were primedwith primers that associate with the single stranded portion of thehairpin. The library was exposed to a Phi-29 type DNA polymerase enzymeunder conditions to allow for primer extension. Primer extension wascarried out for 1 hour at room temperature, then halted. The halting wasdone either with Sr++, EDTA, or a mixture of EDTA and Sr++. Gels wererun to show the extent to which primer extension had occurred and beenhalted. After 4 days at room temperature and at 40° C., gels were run tomeasure the extent of crawling (continued primer extension) during thattime. After this, the complexes were again exposed to conditions forprimer extension including the addition of magnesium. Gels showed thatin most cases, the complexes continued to extend, showing that thecomplexes were still active after storage. FIG. 16 shows the results ofthe stability experiment.

Example 9: Targeting Regions within the Double Stranded Portion of aTemplate

A double stranded template with a size of about 2 Kb having hairpinadaptors at each end was prepared. A series of hook oligomers wasprepared which were complementary to different regions along the doublestranded portion of the template. The hook oligomers targeted differentportions of the double stranded region as depicted in FIGS. 5C and 5D.Hook oligomers had a 3′ A(23) connected to a specific sequence of 13 to15 bases that was complementary to a targeted region within the doublestranded portion. The hook oligomers were designed to target a series ofregions each extending a different number of bases from the primer sitein the hairpin into the double stranded region. The series of hookoligomers was Hook-49, Hook-100, Hook-103, Hook-125, Hook-150, Hook-175,Hook-210, Hook-231, Hook-250, Hook-252, and Hook-317. For example,Hook-100 was a hook oligonucleotide with a 15 base specific captureregion targeted to a portion about 100 bases into the double strandedregion. A series of experiments were run in which a polymeraseassociated with the template was allowed to extend the primer to variousextents by controlling the polymerase reaction conditions and the time.The hook oligomers were used to capture the polymerase-nucleic acidcomplexes that had extended deep enough into the double stranded regionto expose the relevant sequence. The nucleic acid from thepolymerase-nucleic acid complex was isolated and run onto a gel tocharacterize the molecular weight and the amount of product that wascaptured. The experiments demonstrated that hook oligonucleotides can beused to selectively capture specific regions of the template that areopened up by the polymerase by primer extension. FIG. 17 shows theresults of a representative experiment.

Example 10: MagBead Complex Loading Versus Concentration

Polymerase-nucleic acid complexes made with 10 Kb templates SMRTbelltemplates and a phi-29 polymerase were attached to magnetic beads. Asolution of the magnetic beads was dispensed onto a ZMW array chip, anda neodymium magnet was rotated below the chip to bring down the beadsand move them with respect to the surface of the chip. This process wasperformed for 1 hour at four different concentrations of complex: 7.5pM, 15 pM, 30 pM, and 60 pM. For comparison, chip loading by diffusion(no magnetic beads) was performed by exposing the chip topolymerase-nucleic acid complex in solution at 150 pM. Single-moleculesequencing was performed on the samples as described in Eid, J. et al.,Science, 323(5910), 133-138 (2009)). FIG. 18 shows the sequencingresults from the samples. The accuracy of the magnetic bead loadedsamples was higher than that of the sample loaded by diffusion eventhough the diffusion sample was at a much higher concentration.

Example 11: Plasmid Digest Ladder

A plasmid was digested with restriction enzymes to generate a series oftemplates of various insert lengths. The templates were made intocircular (SMRTbell™) templates by the ligation of hairpins onto the endsof the double stranded fragments. The templates were associated with aphi-29 polymerase, and a primer having a polyA tail was hybridized tothe hairpin portion of the template. This sample was coupled to magneticbeads with polyT DNA attached. A solution of magnetic beads wasdispensed onto a ZMW chip and a neodymium magnet below the chip wasrotated to move the beads down and to move them with respect to the chipsurface. Another sample of the polymerase-nucleic acid complex wasloaded onto a ZMW chip by diffusion. The chips were used for singlemolecule sequencing as described in Eid, J. et al., Science, 323(5910),133-138 (2009)). FIG. 19 shows the number of reads that was obtained asa function of the insert length. The data show that with diffusionloading there is a bias toward the smaller templates, whereas thesamples loaded by magnetic bead demonstrated a higher loading of thelarger templates than of the smaller templates. This data show howloading from beads can be extremely beneficial for measuring certaintypes of samples. In many cases, one desires to get the sequence oflarger inserts, but if there is preferential loading of the smallinserts, only a small fraction of the larger inserts are measured. Beadloading allows for efficient loading of these larger templates.

Example 12: Reduction of “Sticking” Pulses

As described above, it is found that in some cases the samples loadedwith magnetic beads show higher accuracy than comparable samples loadedby diffusion. FIG. 20 shows one reason why accuracy is better in thebead loaded samples. Here, the same samples are loaded by diffusion andby magnetic beads. FIG. 20 shows the percent of ZMWs that show GCshowers. GC showers is a name for a phenomenon where a large number ofnon-sequencing peaks are detected. Another name for GC showers is“sticking” which can be caused by having fluorescently labelednucleotide analogs stuck to the surface. We have found that the GCshowers can be an indication of the presence of uncomplexed polymerase.The magnetic bead loaded samples can be rinsed free of excess polymeraseprior to loading onto a chip, while it is difficult to remove suchpolymerases from a polymerase-nucleic acid complex solution produced fordiffusion loading.

Example 13: Magnet Rotation and Chip Coverage

FIG. 21(A) shows an illustration of how the magnet is moved relative tothe chip in one embodiment. The is magnet underneath the chip and isrotated along the dotted line shown. The four representations in FIG.21(A) illustrate how the magnet can be moved closer or farther away fromthe center along the x direction. An experiment was performed in whichthe position of the magnet along the radius (X shown in the figure) andthe distance of the magnet below the chip (Y) was varied. For variousvalues of X and Y, the coverage across the chip was measured. FIG. 21(B)shows a measure of the loading across the chip as X and Y are varied(values in mm). It can be seen that by choosing the appropriate values abroad, relatively even coverage can be obtained.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be clear to one skilledin the art from a reading of this disclosure that various changes inform and detail can be made without departing from the true scope of theinvention. For example, all the techniques and apparatus described abovecan be used in various combinations. All publications, patents, patentapplications, and/or other documents cited in this application areincorporated by reference in their entirety for all purposes to the sameextent as if each individual publication, patent, patent application,and/or other document were individually and separately indicated to beincorporated by reference for all purposes.

We claim:
 1. A method for isolating a nucleic acid from a mixture ofnucleic acids comprising: forming polymerase-nucleic acid complexes bymixing: (a) a polymerase enzyme comprising strand displacement activity,and (b) a mixture of double stranded nucleic acids, each comprising afirst strand and a complementary second strand; initiating nucleic acidsynthesis by the polymerase enzyme to produce a nascent strandcomplementary to the first strand by extending a primer hybridized tothe first strand, thereby displacing a portion of the second strand;halting or reducing the rate of nucleic acid synthesis; hybridizing ahook oligonucleotide to the nucleic acid through a capture region on thehook oligonucleotide that is complementary to at least some of thedisplaced portion of the second strand; and isolating the nucleic acidfrom the mixture of nucleic acids using the hook oligonucleotide.
 2. Themethod of claim 1 wherein the nucleic acid is a circular nucleic acid.3. The method of claim 2 wherein the nucleic acid comprises a doublestranded central region and single stranded hairpin end regions.
 4. Themethod of claim 1 wherein the nucleic acid comprises a linear nucleicacid comprising a linear double-stranded adaptor.
 5. The method of claim1 wherein the hook oligonucleotide also has a retrieval region having asequence that allows the hook oligonucleotide to be bound and retrievedfrom a reaction mixture.
 6. The method of claim 5 wherein the retrievalregion is complementary to a sequence attached to beads, and the beadsare used for the isolation of the nucleic acid.
 7. The method of claim 6wherein the beads are magnetic beads.
 8. The method of claim 6 whereinthe retrieval region comprises poly(A), poly(dA), poly(T) or poly(dT).9. The method of claim 1 wherein the hook oligonucleotide comprises amember of a binding pair providing for isolation of the complexcomprising the hook oligonucleotide bound to the nucleic acid.
 10. Themethod of claim 9 wherein the binding pair comprises biotin,digoxigenin, a protein, or an antibody.
 11. The method of claim 1wherein halting or rate reduction comprises adding a limiting amount ofpolymerase synthesis reagents.
 12. The method of claim 11 wherein thelimiting reagents comprise nucleotides or nucleotide analogs.
 13. Themethod of claim 1 wherein halting or rate reduction comprises addingreagents to stop the polymerization reaction after a time.
 14. A methodfor isolating DNA from a mixture of double stranded DNA comprising;providing a mixture of double stranded DNA; ligating a hairpin to eachend of double stranded DNA in the mixture of double stranded DNA toproduce a population of circular DNA templates having a central doublestranded region and hairpin regions on each end; exposing the populationof circular DNA templates to a primer complementary to the singlestranded portion of a hairpin region of the template and to a DNApolymerase enzyme having strand displacement activity under conditionsin which a population of polymerase-template-primer complexes areformed; initiating polymerase mediated DNA synthesis to extend theprimer, whereby the primer is extended into the double stranded region,displacing a portion of the double strand; halting or reducing the rateof synthesis of DNA; adding to the population of complexes a hookoligonucleotide comprising a capture region complementary to a displacedportion of the double strand under conditions under which hybridizationbetween the hook oligonucleotide and the DNA templates occurs; and usingthe hook oligonucleotide to isolate the circular DNA templates to whichit hybridized.
 15. The method of claim 14 wherein the mixture of doublestranded DNA is produced by fragmenting a DNA sample.
 16. The method ofclaim 14 wherein the hook oligonucleotide also has a retrieval regionhaving a sequence that allows the hook oligonucleotide to be bound andretrieved from a reaction mixture.
 17. The method of claim 16 whereinthe retrieval region is complementary to a sequence attached to beads,and the beads are used for the isolation of the nucleic acid.
 18. Themethod of claim 17 wherein the beads are magnetic beads.
 19. The methodof claim 17 wherein the retrieval region comprises poly(A), poly(dA),poly(T) or poly(dT).
 20. The method of claim 14 wherein the hookoligonucleotide comprises a member of a binding pair providing forisolation of the complex comprising the hook oligonucleotide bound tothe nucleic acid.
 21. The method of claim 20 wherein the binding paircomprises biotin, digoxigenin, a protein, or an antibody.
 22. The methodof claim 14 wherein halting or rate reduction comprises adding alimiting amount of polymerase synthesis reagents.
 23. The method ofclaim 22 wherein the limiting reagents comprise nucleotides ornucleotide analogs.
 24. The method of claim 22 wherein halting or ratereduction comprises adding reagents to stop the polymerization reactionafter a time.
 25. The method of claim 1 wherein the primer is an addedprimer.
 26. The method of claim 1 wherein the primer is a portion of thesecond strand that comprises a nick or gap in the second strand.
 27. Themethod of claim 1 wherein the first strand comprises a hairpin regionand the primer is an added primer hybridized to the hairpin region ofthe first strand.